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How to store a Task in a dictionary for later execution in C#
I'm trying to store a Task in a dictionary for later execution with no success:
If I declare the function as
Then I can do:
But I need the function to be async. And don't know how to invoke it. I've tryed:
with no success.
- 3 You really should not store Task or Task<T> objects to be run later, you should be storing Func<Task> or Func<Task<T>> objects. Also, there is never a reason to do new Task(... unless you are writing a task schedueller, You should only work with "hot tasks". – Scott Chamberlain Commented Nov 7, 2016 at 16:38
- Thank you very much @ScottChamberlain. That did it. Since you were the first one responding, if you want me to accept your answer, please post one. If you don't, I will accept Servy's. – mattinsalto Commented Nov 7, 2016 at 16:55
- 1 Go accept Servy's – Scott Chamberlain Commented Nov 7, 2016 at 17:13
If you want to be able to start an asynchronous operation later then you don't want to store a Task , you'll want to store a Func<Task> (or, in your case, a Func<Task<string>> , so that, when you want to start the operation, you can invoke the function, producing a Task that represents its completion.
- 1 @aledpardo If you want to store something that can start a task later, you store a Func<Task> , and then you execute the function when you want to start it, giving you a Task . – Servy Commented Jun 6, 2017 at 20:32
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AOverflow.com
How to return a string from a Task<string>?
I am performing an HttpRequest. It turns out that when I return the response and show it to work on it, it shows me this:
My code is:
It turns out that if I show in the console before returning the responseString , the information obtained is displayed correctly.
I am calling it from a method in the MainWindow.xaml.cs connected to a button.
How can I do so that when I return I get the information I need?
One option is to use the method Task.FromResult<TResult> , where you would only change return responseString; to return Task.FromResult<responseString>; , leaving your code as follows:
Or in your case, change the line from HttpResponseMessag response = ... to HttpResponseMessage response = await Task.FromResult<string>(httpClient.PostAsync(new UrlHttpRequest().UrlReadClients, content));
OP comments:
I'm instantiating the method, maybe that's where I have the problem. i call it like that Console.WriteLine(new Client().ReadClients(1))
Indeed, that is the problem. Since ReadClients it's a method async , to get the return value, you need to use await :
However, if you are making the call from main for example, which you cannot mark as async , then you would have to make the call like this:
But it should be noted that as of C# 7.1 it is possible to mark the main with async in the following way:
Edit based on the code you added to your question
If you are trying to make the call from an event handler, you must add async to the event handler's signature and run ReadClients with await :
...or, if you fall back to your original version of ReadClients which directly returns the desired result (probably a good idea), it's basically the same:
I found another way to get the information. To make it easier I created a new string variable inside the class to which I assign the public string s = responseString; so when I instantiate it I do it in a new way.
This way I get the content that returns.
How to Return a Value from Task in C#
Back to: C#.NET Tutorials For Beginners and Professionals
How to Return a Value from Task in C# with Examples
In this article, I am going to discuss How to Return a Value from a Task in C# with Examples. Please read our previous article where we discussed Task in C# with Examples. At the end of this article, you will understand How to Return a Value from a Task in C# with examples.
How to Return a Value from a Task in C#?
The .NET Framework also provides a generic version of the Task class i.e. Task<T>. Using this Task<T> class we can return data or values from a task. In Task<T>, T represents the data type that you want to return as a result of the task. With Task<T>, we have the representation of an asynchronous method that is going to return something in the future. That something could be a string, a number, a class, etc.
Example to Understand Task<T> in C#:
Let us understand this with an example. What are we going to do is, we are going to communicate with a Web API that we are going to build and we will try to retrieve the message that we receive from the Web API.
Creating ASP.NET Web API Project
Open Visual Studio and creates a new ASP.NET Web API Project. If you are new to ASP.NET Web API, then please have a look at our ASP.NET Web API Tutorials. Here, we are creating an Empty Web API Project with the name WebAPIDemo. Once we created the Web API Project then add a Web API Controller with the name GreetingsController inside the Controllers folder. Once you add the GreetingsController then copy and paste the following code inside it.
Now, run the Web API application, and you can access the GetGreetings resource using the URL api/greetings/name as shown in the below image. Please note the port number, it might be different in your case.
Once you run the Web API Project, then you can access the above resource from anywhere. You can access it from a web browser, you can access using postman and fiddler, and you can also access it from other web, windows, and console application. In our example, we are going to access this from our Console application.
The idea is that since the Web API is external to our Console application. So, talking to the Web API is an IO operation, which means that we will have to use or we should use Asynchronous Programming.
Calling Web API HTTP Request from Console Application
Now, we will make an HTTP Request to the Web API (External Resource) from our Console Application. Please copy the endpoint address of the Web API. And then modify the code as follows. You need to replace the port number on which your Web API application is running. In the below example, we are making an asynchronous call to the Web API.
Output: Before running the console application, please make sure your web application is running. Once your Web API Application is running, then run the console application. It will ask you to enter your name. Once you enter the name, press the enter key and you will see the following output.
The point that you need to remember is that if you are writing any asynchronous method, then you can use Task as the return type if it is not returning anything or you can use Task<T> when your method returns something. Here T can be anything like string, integer, Class, etc.
And we also saw that by using await, we are suspending the execution of the current thread. So we are freeing the thread so that the thread can be used in other parts of the application. And once we have a response, for example, from our Web API, then it will use the thread again to execute the rest part of the method.
C# Task with Errors:
So far, every task that we have executed has been completed successfully. And, in real life, this may not be the case always. Sometimes errors will happen. For example, maybe we were wrong in the URL. In this case, we will get a 404 error. Let us understand this with an error. In the URL, I have changed greetings to greetings2 as shown in the below code. Further, I have included the response.EnsureSuccessStatusCode(); statement to throw 404 error.
Output: With the above changes in place now we run the application, before that make sure the Web API Application is running. Enter the name and press the enter button as shown in the below image.
Once you enter your name and press the enter button, then you will get the following unhandled exception.
Please observe here we are getting 404 Not Found HttpRequestException. This is a bad user experience. The user should not see this message. If any exception occurred then instead of showing the exception details, we should show some generic error message. Let us see how we can do this. Within the SomeMethod we need to use the Try and Catch block to handle the unhandled exception which is shown in the below example.
Now, we are not getting that exception rather we seeing some generic message on the console. This is different than having an unhandled exception. So, here we completely control what was going to happen if we get an exception.
What happens if we omit the await keyword while calling the Greetings method?
Something that you need to keep in mind is that if you don’t await the task, then the exception will not be thrown to the caller method i.e. the method from where we called the async method. In our example, it will not throw the exception to the SomeMethod. Let’s see that. Let’s remove the await keyword and the printing of the greeting statement inside the SomeMethod as shown in the below example and run the application.
Now, when you run the application, you will not get the exception. You will get the following output which does execute the catch block.
Why we did not get the exception?
Please have a look at the below image. When an Exception has occurred inside an async method, that exception is encapsulated inside the Task.
If you want to unwrap the exception then you need to use await as shown in the below image. If you are not using await, then you will never get the exception.
Note: We can catch exceptions by using a simple try-catch block. But if we never await the task, then even if we have an exception, the exception is not going to be thrown. So, if you want to be notified about the exceptions that you may have, you need to await the task.
Example to Understand How to Return Complex Type Value From a Task in C#:
In the below example, we are returning a Complex type.
In the next article, I am going to discuss How to Execute Multiple Tasks in C# with Examples. Here, in this article, I try to explain How to Return a Value from a Task in C# with Examples. I hope you enjoy this Task that Returns a Value in C# with Examples article.
About the Author: Pranaya Rout
Pranaya Rout has published more than 3,000 articles in his 11-year career. Pranaya Rout has very good experience with Microsoft Technologies, Including C#, VB, ASP.NET MVC, ASP.NET Web API, EF, EF Core, ADO.NET, LINQ, SQL Server, MYSQL, Oracle, ASP.NET Core, Cloud Computing, Microservices, Design Patterns and still learning new technologies.
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Master C# Dictionary: Complete guide
May 13, 2023 | .NET , C#
Are you ready to dive into the world of C# dictionaries and optimize your code with fast and efficient data storage techniques? Let’s get our hands dirty with some advanced C# programming!
Introduction to C# Dictionary
C# dictionaries are powerful data structures that offer many benefits and can improve the way we write code. In this section, we will explore the concept of dictionaries in C# and some of their most common use cases.
What is a Dictionary in C#
A dictionary in C# is a collection of key-value pairs, where each key is unique and associated with one value. It can be thought of as a sort of virtual table where you can quickly look up values based on their keys. They are a part of the System.Collections.Generic namespace and are a central component when working with collections in C#.
C# Dictionary Example
Now that we understand what a dictionary in C# is, let’s take a closer look at an example of how to declare and work with a dictionary.
In the code snippet above, we create a dictionary called phonebook with a string representing the name, and an integer representing their phone number. We then add two people, John and Jane, along with their phone numbers.
Variants of C# Dictionaries
C# offers several variants of dictionaries that provide different capabilities, such as sorted collections or collections optimized for concurrent access. Some examples include SortedDictionary<TKey, TValue> , ConcurrentDictionary<TKey, TValue> , and ReadOnlyDictionary<TKey, TValue> .
Creating and Initializing C# Dictionary
Creating and initializing a dictionary is a fundamental aspect of working with this data structure in C#. In this section, we will discuss different ways to create and initialize dictionaries and dive into various techniques available for dictionary initialization.
Creating a Dictionary in C#
To create a dictionary in C#, we first define the dictionary and then initialize it. The following code example demonstrates how to create a dictionary with keys and values using various techniques.
Initializing a Dictionary with Values
When you create a dictionary, you may also want to initialize it with some predefined key-value pairs. In C#, you can utilize the dictionary initializer to accomplish this task.
Dictionary Initializers in C#
Starting with C# 6.0, you can use dictionary initializers to populate your dictionaries more elegantly. This syntax allows you to add keys and values by separating the key and the value with a colon, making it more apparent that you are working with key-value pairs.
Creating a New Dictionary
When you need to create a new dictionary from an existing one, you can use the ToDictionary method or simply create a new instance by passing the existing dictionary as a constructor argument.
C# to Dictionary Example
Converting existing data structures like lists or arrays into dictionaries can be quite useful. With LINQ, you can easily convert data structures into a dictionary using the ToDictionary method.
Working with C# Dictionary
In this section, we will take a closer look at how to work with dictionaries in C#. We’ll learn about some of the essential operations, such as adding and retrieving items, setting values, and working with keys.
Understanding Dictionary Keys
When working with dictionaries, it’s crucial to understand the importance of unique keys. Keys allow you to quickly access the values stored in the dictionary, while also ensuring that each entry has a unique identifier.
- Keys must be unique within the same dictionary.
- Keys cannot be null.
- Adding a duplicate key to a dictionary will result in an ArgumentException .
- You can iterate through the keys in a dictionary using the Keys property.
Adding Items to a Dictionary
To add items to a dictionary, we can use the Add method, which takes two arguments: the key and the associated value. Keep in mind that the keys must be unique and cannot be null.
Getting Values from a Dictionary
To retrieve a value from a dictionary, we can use the .TryGetValue method, which takes two arguments: the key and an out parameter for the associated value.
A more straightforward way to access the dictionary value by key is using the indexer. However, if the key is not found, it will throw a KeyNotFoundException .
Retrieving Key-Value Pairs from a Dictionary
When we need to work with both keys and values, we can iterate through the key-value pairs using the KeyValuePair<TKey, TValue> structure.
IDictionary Interface in C#
The IDictionary<TKey, TValue> interface is a more generic representation of dictionaries, which you can implement using different underlying data structures. By working with the IDictionary interface, your code can be more flexible and adaptable to other collection types.
Setting Dictionary Values
To update the values associated with dictionary keys, you can use the indexer notation.
C# Dictionary Methods and Techniques
C# dictionaries offer various methods and techniques that help us perform different operations efficiently. In this section, we will discuss these techniques, such as initializing a dictionary with specific values, creating dictionaries with values, and using dictionary keys in different ways.
Common Dictionary Methods
Some commonly used dictionary methods include:
- Add – Adds a key-value pair to the dictionary.
- ContainsKey – Determines if the dictionary contains a specific key.
- TryGetValue – Retrieves the value of the given key, returning true if the key is present, false otherwise.
- Remove – Removes the key-value pair identified by the given key.
- Clear – Removes all keys and values from the dictionary.
Initializing a Dictionary with Specific Values
To initialize a dictionary with specific values, you can use the collection initializer syntax.
Working with Key-Value Pairs
To work with key-value pairs in a dictionary, iterate through the dictionary using a KeyValuePair<TKey, TValue> structure.
Converting a Dictionary to a String
To convert a dictionary into a string representation, you can use a StringBuilder or string.Join method.
Or, with string.Join method:
Using TryGetValue Method
The TryGetValue method can be quite useful when working with dictionaries, as it allows you to efficiently check for the presence of a key, and, if present, retrieve its value without causing an exception.
Ordering a Dictionary
To reorder a dictionary based on key or value, we can use LINQ methods like OrderBy and OrderByDescending .
Creating and Initializing a Dictionary with Values
You can create and initialize a dictionary with pre-defined values using the dictionary initializer syntax, as shown below:
Accessing Dictionary Keys
To access the keys of a dictionary or perform operations only on the keys, you can use the Keys property.
In conclusion, C# dictionaries are a powerful tool for developers, offering efficient data storage and retrieval capabilities. From creating and initializing dictionaries to managing key-value pairs, the rich set of methods and techniques available make working with C# dictionaries a breeze. Remember to utilize the features discussed in this article whenever you work with dictionaries and enjoy the perks of high-performance and organized code!
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C#, Dictionaries and Multithreading
A short lesson about using shared Dictionaries in multithreaded C# environments.
8 years ago on Dev , .NET , CS — Go back home
Today we encountered a very strange issue. One of our clients' ASP.NET based website was running really slow, at least on one of the two servers it is running on. And by slow I mean it was almost unreachable.
Some inspection on the issue showed us that the CPU load on the affected server was somewhere between 95% and 100%. The uptime of the site on this server was some days. And before that the site was already running for some weeks without any problems. So why should something like that happen out of a sudden? Well, as we later found out, it had something to do with thread safety of Dictionaries .
But first things first: ASP.NET websites running under IIS are multithreaded by default. What this means is that the execution of the code is asynchronous and simultaneous.
What happened in our case is the following: We have a class (let's call it SomeContainer ) that has a static property that is of type Dictionary . Each time this property is accessed it is referencing the exact same instance - that's just what we want to achieve with this approach. But the way dictionaries are designed they were not meant for this scenario of reading and writing concurrently.
When the entries in a Dictionary change while they are being iterated over, an InvalidOperationException will be thrown saying that the Collection was modified; enumeration operation may not execute. and the iteration steps on to the next entry - where the Exception is thrown again - and so on. You get the point. This happens endlessly causing a deadlock. The high CPU load only seems to be a symptom, not the problem itself.
To illustrate what exactly happened let's take a look at this code snippet:
This is our container. As you can see it's got one initial entry so there's something to loop over.
The easiest way to provoke this error is to add a new entry while iterating over the dictionary.
This will immediately throw an InvalidOperationException . And another one. And even another one. Have a look at the CPU load of this program.
So much for theory. Next we'll construct the rare case that two threads read and write to the dictionary at the same time. For this I'm simply spawning two threads, one that iterates over the dictionary and one that adds new entries to it.
As this case is really rare, I've added some concurrency so that the chance for the threads to access at the same time increases.
For this test case we don't need an initial entry in the dictionary, so let's remove it:
And finally:
You may need to run this snippet multiple times for the error to show up... As I said before, it's somewhat shy and only shows up quite seldom :)
So what did we do to prevent this happening again? Luckily, there's a thread save implementation for dictionaries called ConcurrentDictionary that can be used as a drop in replacement.
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- TECHNOLOGIES
- An Interview Question
A comprehensive overview of Dictionary in C#
- Aug 25, 2023
- Other Artcile
Dictionaries are essential tools for managing data efficiently through key-value pairs, offering rapid retrieval and manipulation. The Dictionary<TKey, TValue> class empowers developers to effectively add, access, and update items, enhancing performance and problem-solving capabilities.
What is a Dictionary?
A dictionary is a collection type in C# that allows you to store key-value pairs. Each key in a dictionary is unique, and it is used to retrieve the associated value. This makes dictionaries an efficient data structure for looking up and retrieving values based on their keys. Think of it like a real-world dictionary where you look up words and find their meanings. In a C# dictionary, you use a key to look up a specific value.
Key Characteristics of Dictionaries
- Key-Value Pair: Each element in a dictionary consists of a key and an associated value. The key is used to uniquely identify the value.
- Uniqueness: Keys within a dictionary must be unique. You cannot have duplicate keys.
- Efficient Lookups: Dictionaries provide fast lookup times for retrieving values based on their keys. This makes them suitable for scenarios where quick access to values is essential.
- Dynamic Size: Dictionaries can grow or shrink as you add or remove key-value pairs.
- No Guarantee of Order: Dictionaries do not guarantee a specific order of key-value pairs. The order is not preserved as it is in arrays or lists.
"Imagine you're the manager of a small online store, and you want to keep track of products and their prices. You decide to use a digital system to handle this information. In this system, each product's name is like a key, and the price of the product is the value associated with that key. Similar to how customers would search for a product by its name to find its price on your website, in a C# dictionary, you use the product name as the key to quickly retrieve its price."
In this scenario, the online store's product database functions like a dictionary, where product names (keys) are utilized to efficiently find and display the corresponding price (value).
Creating Dictionaries
Creating dictionaries in C# involves declaring, initializing, and populating them with key-value pairs.
Example 1. Declaring and Initializing Using Object Initializer.
Example 2. Adding Key-Value Pairs After Initialization.
Example 3. Using Type Inference with var.
Example 4. Creating an Empty Dictionary and Adding Later.
Why Use Dictionaries?
Dictionaries are particularly useful when you need to quickly lookup values based on some unique identifier (the key). Here are some scenarios where dictionaries shine:
- Caching: Storing cached data where the key is the input and the value is the computed result.
- Data Indexing: Indexing large datasets for faster retrieval.
- Configurations: Storing configuration settings where the key represents a setting name and the value is the corresponding value.
- Mapping: Creating a mapping between two related pieces of data.
Dictionary Types in C#
1. Dictionary<TKey, TValue>
The Dictionary<TKey, TValue> class is the most commonly used dictionary type in C#. It allows you to store key-value pairs, where TKey represents the type of keys, and TValue represents the type of values. This class provides fast lookups and is suitable for general dictionary scenarios.
2. SortedDictionary<TKey, TValue>
The SortedDictionary<TKey, TValue> class is similar to the `Dictionary<TKey, TValue>`, but it maintains the keys in sorted order. This can be useful when you need key-value pairs sorted by keys.
3. ConcurrentDictionary<TKey, TValue>
The ConcurrentDictionary<TKey, TValue> class is designed for multi-threaded scenarios where concurrent access to the dictionary is required. It provides thread-safe methods for adding, updating, and retrieving values.
4. ImmutableDictionary<TKey, TValue>
The ImmutableDictionary<TKey, TValue> class is part of the System.Collections.Immutable namespace and provides an immutable (unchangeable) dictionary. This is useful when you want to ensure that the dictionary's state does not change after it's created.
5. HybridDictionary
The HybridDictionary class combines the functionality of a list and a dictionary. It automatically switches between using a ListDictionary and a Hashtable based on the number of elements.
Note. These are some of the commonly used dictionary types in C#. The choice of dictionary type depends on your specific use case, including performance requirements, threading considerations, and whether you need ordered keys.
Dictionary Methods and Properties
Some important methods and properties of the `Dictionary<TKey, TValue>` class in C#:
1. Add(TKey key, TValue value)
The Add method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to add a new key-value pair to the dictionary. The method requires a key and a value as parameters and adds them to the dictionary. If the key already exists, an exception might be thrown.
2. Remove(TKey key)
The Remove method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to remove a key-value pair from the dictionary based on the provided key. If the key is found, the associated key-value pair is removed from the dictionary.
3. TryGetValue(TKey key, out TValue value)
The TryGetValue method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to retrieve the value associated with a given key from the dictionary. Unlike direct indexing (dictionary[key]), TryGetValue does not throw an exception if the key is not found; instead, it returns a boolean indicating success and an "out" parameter for the retrieved value.
4. ContainsKey(TKey key)
The ContainsKey method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to check whether a specific key exists in the dictionary or not. It returns a boolean indicating whether the key is present in the dictionary.
5. ContainsValue(TValue value)
The ContainsValue method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to check whether a specific value exists in the dictionary or not. It returns a boolean indicating whether the value is present in the dictionary.
The Clear method in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, is used to remove all key-value pairs from the dictionary, effectively clearing its contents.
The Keys property in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, allows you to get a collection of all the keys present in the dictionary.
The Values property in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, allows you to get a collection of all the values present in the dictionary.
The Count property in various dictionary implementations, like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, allows you to get the number of key-value pairs present in the dictionary.
2. Item[TKey key]
The Item[TKey key] syntax, also known as the indexer, in various dictionary implementations like Dictionary<TKey, TValue>, ConcurrentDictionary<TKey, TValue>, and SortedDictionary<TKey, TValue>, allows you to access or modify the value associated with a specific key in the dictionary.
3. Comparer
The Comparer property in the SortedDictionary<TKey, TValue> class is used to get the IComparer<TKey> implementation that is used to compare keys in the sorted dictionary.
The Comparer property allows you to access the comparer that determines the order of keys in the sorted dictionary. If you don't provide a custom comparer when creating a SortedDictionary<TKey, TValue>, it will use the default comparer for the key type TKey.
Iterating Through Dictionaries
Using foreach loop with KeyValuePairs: This is the most common and convenient way to iterate through a dictionary's key-value pairs.
Example of Building a social media application and you want to keep track of users and their follower counts using a dictionary.
Simple console-based implementation that allows you to add items to the inventory, view the current inventory, and search for items.
Example to manage household items and their quantities using Dictionary.
Dictionaries are powerful tools for managing data in various scenarios, and understanding their usage can greatly enhance your programming capabilities.Dictionaries are a fundamental data structure that provide an efficient and flexible way to store, retrieve, and manage data using key-value pairs.Dictionaries are dynamic data structures that store key-value pairs, offering fast retrieval and manipulation of data. Utilizing the Dictionary<TKey, TValue> class, developers can efficiently add, access, and update items. Dictionaries are valuable for tasks like caching, indexing, and lookups, enhancing performance. They provide a crucial tool for managing data associations, combining keys and values for powerful data representation.
As you continue your C# programming Learning journey, a solid understanding of dictionaries will empower you to solve various challenges effectively.
If you encounter any issues or have further questions, feel free to let me know, and I'll be glad to assist!
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Creating a dictionary of strings and list of strings
I have a List of objects, List<myObject> objectList , and each object contains a List of strings like so:
I'm trying to merge the lists into a dictionary of: Dictionary<string, List<string>> , where the final results will look like so:
Here is what I have done, and it does work:
Is there a better way to do this, where I don't have to use two foreach loops? Is there a way to do it without using any foreach loops?
I have tried using lambda but this is as far as I got:
The problem is that the expression keeps overwriting any existing entries in the dictionary each time I iterate through the loop.
Is there a way to keep the existing entries, groupby the Key and add to the existing list of values but don't duplicate any values?
I think the answer lies in this part of the expression:
but I'm not 100% sure.
- 1 \$\begingroup\$ Did you try SelectMany? \$\endgroup\$ – Mike Cheel Commented Nov 27, 2013 at 16:28
Just get rid of the outer foreach as well:
SelectMany retrieves all the StringList s and flattens them into one single list:
- \$\begingroup\$ I think you want a distinct in there as well so that A is 1,2 rather than 1,2,2 . \$\endgroup\$ – Chris Commented Nov 27, 2013 at 16:39
- 1 \$\begingroup\$ x => x.Select(g => g[1]).Distinct().ToList() \$\endgroup\$ – Ahmed KRAIEM Commented Nov 27, 2013 at 16:40
- 1 \$\begingroup\$ Note that Distinct destroys order , so it might be 1,2 or 2,1 . \$\endgroup\$ – Tim S. Commented Nov 27, 2013 at 16:48
- \$\begingroup\$ @Heinzi - Thanks so much! This works perfectly! Much simpler and more elegant. For the life of me, I couldn't figure this out...a million times thank you! \$\endgroup\$ – user3042376 Commented Nov 27, 2013 at 16:59
- 1 \$\begingroup\$ @Groo Like a comment at my link says, "what you say could be true [that it preserves first-found order], but it would be a bad idea to rely on that behavior" (because it is documented as being an unordered sequence, you should treat it as such). Also, I don't know that objectList had it sorted to begin with, the example just happens to have it so (I think). \$\endgroup\$ – Tim S. Commented Nov 27, 2013 at 17:07
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.Net Tips – Xml Serialize or Deserialize Dictionary in C#
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If you try to serialize/deserialize a type which uses the generic Dictionary<TKey, TValue> type with the XmlSerializer then you’ll get an InvalidOperationException , for instance:
Here’s my class:
I’ll get an InvalidOperationException with an inner exception of type NotSupportedException when I do this:
And the error message of the NotSupportedException is:
“Cannot serialize member MyClass.MyDictionary of type System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], because it implements IDictionary.”
The reason why Dictionary<TKey, TValue> is not supported by the XmlSerializer is that types such as Dictionary , HashTable , etc. needs an equality comparer which can’t be serialized into XML easily and won’t be portable anyhow. To get around this problem, you generally have two options, in the order of my personal preference:
Use DataContractSerizalizer
The easiest, and cleanest solution is to switch over to data contract serialization, and the only change required would be to mark your class with the DataContractAttribute and mark the properties you want to serialize with the DataMemberAttribute . My class would now look like this:
And to serialize it into Xml, just use the DataContractSerializer and write the output with a XmlTextWriter like this:
Use a custom Dictionary type
The alternative is to create your own Dictionary implementation which is Xml serializable. See the references section for an implementation Paul Welter has created.
References:
XML Serializable Generic Dictionary
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16 thoughts on “.Net Tips – Xml Serialize or Deserialize Dictionary in C#”
great stuff! exactly what i needed. thx for your work and posting.
serializer.WriteObject(writer, marketplace);
where does marketplace come from?
@Nanek – imagine ‘marketplace’ being an instance of ‘MyClass’, looks like I missed out a line or two of the code snippet
Thank you for sharing this.
Thanks for the information.
Just thought I’d add – you need to add a reference to System.Runtime.Serialization for this, and it is only available in .NET 3 upwards.
Pingback: [C#] Dictionary ?? Xml « Develope note
Thank you very much for this post. I really needed something like this.
Thank you. This helped me serialize an object not serializable with standard xml serializer.
Great! I’d add that for DataContractSerizalizer a reference to System.Runtime.Serialization is necessary. Thnaks, guy!
Thanks. how do you then read/deserialize it?
In .Net 4.5.1 I am getting
{“Type ‘System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]’ with data contract name ‘ArrayOfKeyValueOfstringstring: http://schemas.microsoft.com/2003/10/Serialization/Arrays ‘ is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types – for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.”}
Here is the class I am trying to serialize
[DataContract]
public class KeywordFunctionMap
[DataMember]
public Dictionary Map { get; set; }
// need a parameterless constructor for serialization
public KeywordFunctionMap()
Map = new Dictionary();
I just put together a Console app in .Net 4.5.1 and it worked as expected, see https://gist.github.com/theburningmonk/3b1f1a33ba616bda7474
Pingback: Serialising complex types in C#
Thanks! How do I make desrialize tto Dicitonary?
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C# (CSharp) PickNow Examples
Cannot convert from 'System.Threading.Tasks.Task' to 'System.Collections.Generic.Dictionary<string,string>'
I believe I might just have the syntax wrong but what I'm trying to do is create a task that runs after another task is finished.
I have a task for each array of 100 in a list. It starts a new thread passing that array into a method. The method returns a dictionary when it finishes. I'm trying to create a task to run after the method is finishes where it passes the returned dictionary to a separate method that does some more work.
solution1 5 ACCPTED 2014-12-25 06:52:53
I would advise not to use ContinueWith at all if you're already using await . await 我建议不要使用 ContinueWith 。--> The reason is the verbosity you end up with in your code.
Instead, use await where possible. await 。--> The code ends up like this:
Note the use of Task.Run instead of Task.Factory.StartNew . Task.Run 代替 Task.Factory.StartNew 。--> You should use that instead. More on that here 这里 更多-->
Edit: 编辑： -->
If you need to execute this code once every 24 hours, add a Task.Delay and await on it : Task.Delay 并 await 它：-->
Edit 2: 编辑2： -->
The reason your code isn't working is because startDownload is async void , and you're not awaiting on it. startDownload 为 async void ，并且您没有等待它。--> Thus your while loop keeps iterating regardless of your Task.Delay . while 无论您的 Task.Delay 如何，您的 while 循环都会保持迭代。-->
Because you're inside a Console Application, you can't await because Main methods cant be async. await 因为 Main 方法不能异步。--> So to work around that, change startDownload to be async Task instead of async void , and Wait on that returned Task . startDownload 更改为 async Task 而不是 async void ，然后 Wait 返回的 Task 。--> Do note that using Wait should almost never be used, expect for special scenarios (such as the one while running inside a console app): 几乎 不应该使用 Wait ，除非有特殊情况（例如在控制台应用程序中运行时）：-->
Also note that mixing Parallel.Foreach and async-await isn't always the best idea. Parallel.Foreach 和 async-await 并非总是最好的主意。--> You can read more on that in Nesting await in Parallel.ForEach 在Parallel.ForEach 中的 嵌套等待中 了解更多信息。-->
solution2 2 2014-12-25 05:59:04
You'll see that ContinueWith takes as Task as argument. ContinueWith 将Task作为参数。-->
Also, from working through your logic in our comments, it looks like you'll need to update your code. You are trying to run the Downloads.updateSymbolsInDB once on the result of all WhenAll tasks are completed. WhenAll 任务的结果运行一次 Downloads.updateSymbolsInDB 。--> In your case it looks like you'll need
Note that I also used the Task<TResult> to initiate, so that the ContinueWith is strongly typed to provide the Dictionary<string, string> i.Result . Task<TResult> 进行初始化，因此对 ContinueWith 进行了强类型化，以提供 Dictionary<string, string> i.Result 。-->
Also note that you'll want to revisit what I'm doing in the i.Result.Aggregate logic. i.Result.Aggregate 逻辑中的操作。--> You'll need to update that to be correct to your case. Additionally, it may be worth revisiting to see if it is more efficient to just call Downloads.updateSymbolsInDB multiple times, or if the Aggregate call is the better choice. Downloads.updateSymbolsInDB 是否更有效，或者 Aggregate 调用是否是更好的选择。-->
(Last note: the end result is a void , so the await isn't assigned.) void ，因此未分配等待。）-->
In review, there were a number of problems with your code (meant as a statement, not an accusation - no offense intended). Your tasks created via Task.Factory.StartNew for your WhenAll method had no return value, thus were just basic Task objects, and were in fact throwing away the results of the Downloads.getHistoricalStockData work. Task.Factory.StartNew 为 WhenAll 方法创建的任务没有返回值，因此只是基本的 Task 对象，实际上正在丢弃 Downloads.getHistoricalStockData 工作的结果。--> Then, your ContinueWith needed to work with the Task<TResult> of the WhenAll , where TResult will be an array of the return value of each task within WhenAll . ContinueWith 需要与 WhenAll 的 Task<TResult> WhenAll ，其中 TResult 将是 WhenAll 中每个任务的返回值的 WhenAll 。-->
EDIT: As was asked how the code could be updated if Downloads.updateSymbolsInDB were to be called multiple times, it could simply be done as: 编辑： 如被问到如果要多次调用 Downloads.updateSymbolsInDB ，如何更新代码，可以简单地通过以下方式完成：-->
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HttpClient 任务返回 System.Threading.Tasks.Task`1[System.String] 而不是预期的 JSON
问题描述 投票：0 回答：2.
我正在尝试访问以下代码应生成的 JSON 响应：
在 Postman 或 Javscript 中执行此操作时，结果是正确的，所以我想我无法以正确的方式访问任务字符串:-)
任何为我指明正确方向的帮助将不胜感激。
您的代码包含多个错误。
如果您不确定自己在做什么，请不要使用
是的，正如上面评论中所维护的，您必须使用
最后，是时候向 异步编程 问好了。 :)
无法正确打印的原因是 GetResponseString() 返回的是
要打印您期望的 json，您需要编写...
或者，您可以将方法切换为仅返回
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Converting String List Dictionaries in Python for Excel File Reading with Bunch
Abstract: In this article, we will discuss how to convert string list dictionaries to a usable format while reading an Excel file using the Python library Bunch.
Converting String List Dictionaries from Python Excel File Reading Bunch
In this article, we will discuss how to convert string list dictionaries from Python Excel file reading bunch. We will cover the key concepts and provide detailed context on the topic. The article will be at least 800 words long and will include subtitles, paragraphs, code blocks, and references.
What is a Bunch in Python?
A bunch in Python is a simple class that allows you to group related data together. It is often used as a convenient way to pass around multiple related values without having to define a full-blown class. A bunch is essentially a dictionary with named keys, making it easier to access the values.
Reading an Excel File in Python
To read an Excel file in Python, we can use the popular openpyxl library. This library allows us to read and write Excel files in Python. Here's an example of how to read an Excel file using openpyxl :
In this example, we first import the openpyxl library. We then load the workbook using the load\_workbook() function and select the active sheet. Finally, we iterate over the rows and cells in the sheet, printing the value of each cell.
Converting String List Dictionaries from Excel File
Once we have read the Excel file, we can convert the data into string list dictionaries. Here's an example of how to do this:
In this example, we first load the Excel file and select the active sheet, just like in the previous example. We then create an empty list to store the data. We iterate over the rows in the sheet, and for each row, we create a new dictionary to store the data. We iterate over the cells in the row, and for each cell, we add an entry to the dictionary using the column letter as the key and the cell value as the value. Finally, we append the dictionary to the data list.
In this article, we have discussed how to convert string list dictionaries from Python Excel file reading bunch. We have covered the key concepts, including what a bunch is in Python and how to read an Excel file using the openpyxl library. We have also provided detailed code examples on how to convert the Excel file data into string list dictionaries.
- openpyxl Documentation
- Reading and Writing Excel Files in Python with openpyxl
Tags: : Python Excel Bunch Dictionary String List
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You do not want to throw an Exception in after you started some tasks, better validate first. private static async Task<Dictionary<string, string>> GetETagsAsync(List<string> feedsLink) { ValidateFeedsLinkFormat(feedsLink); var _eTags = feedsLink.ToDictionary(f => f, f => GetETagAsync(f)); await TaskEx.WhenAll(_eTags.Values); var eTags = _eTags.ToDictionary(k => k.Key, k => k.Value.Result ...
12. If you want to be able to start an asynchronous operation later then you don't want to store a Task, you'll want to store a Func<Task> (or, in your case, a Func<Task<string>>, so that, when you want to start the operation, you can invoke the function, producing a Task that represents its completion. @aledpardo If you want to store something ...
HttpResponseMessage response = await httpClient.PostAsync(new UrlHttpRequest().UrlReadClients, content); string responseString = await response.Content.ReadAsStringAsync(); return responseString; It turns out that if I show in the console before returning the responseString, the information obtained is displayed correctly. I am calling it from ...
Using this Task<T> class we can return data or values from a task. In Task<T>, T represents the data type that you want to return as a result of the task. With Task<T>, we have the representation of an asynchronous method that is going to return something in the future. That something could be a string, a number, a class, etc.
C# (CSharp) Func ,Task> - 35 examples found. These are the top rated real world C# (CSharp) examples of Func ,Task> extracted from open source projects. You can rate examples to help us improve the quality of examples. static AppFunc ShowFormValues(AppFunc next) return env =>. var req = new Request(env);
Add - Adds a key-value pair to the dictionary. ContainsKey - Determines if the dictionary contains a specific key. TryGetValue - Retrieves the value of the given key, returning true if the key is present, false otherwise. Remove - Removes the key-value pair identified by the given key.
C#, Dictionaries and Multithreading. Today we encountered a very strange issue. One of our clients' ASP.NET based website was running really slow, at least on one of the two servers it is running on. And by slow I mean it was almost unreachable. Some inspection on the issue showed us that the CPU load on the affected server was somewhere ...
Converting Dictionaries to Strings: To convert a dictionary to a string in C#, we have several approaches. One common method is to iterate through the key-value pairs and concatenate them into a string using a delimiter. Let's consider the following dictionary: Dictionary<string, int> myDictionary = new Dictionary<string, int>() { { "apple", 5 ...
Now, let's explore the process of converting strings into dictionaries. One common approach is to parse the string and extract key-value pairs, then populate a dictionary with this data. Here's a step-by-step breakdown of how this can be achieved: Split the String: Start by splitting the input string into individual key-value pairs.
1. Dictionary<TKey, TValue>. The Dictionary<TKey, TValue> class is the most commonly used dictionary type in C#. It allows you to store key-value pairs, where TKey represents the type of keys, and TValue represents the type of values. This class provides fast lookups and is suitable for general dictionary scenarios.
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To create and initialize a list in C#, you can use the List<T> class, where T represents the data type of the elements you want to store. Here's how to create and initialize a list of integers ...
Finally, the stopwatch outputs are printed in milliseconds and here are the results. As you can see, retrieving an item from a dictionary is massively more performant and efficient than doing the ...
If you try to serialize/deserialize a type which uses the generic Dictionary<TKey, TValue> type with the XmlSerializer then you'll get an InvalidOperationException, for instance:. Here's my class: public class MyClass { // need a parameterless constructor for serialization public MyClass() { MyDictionary = new Dictionary<string, string>(); } public Dictionary<string, string> MyDictionary ...
C# (CSharp) PickNow - 5 examples found. These are the top rated real world C# (CSharp) examples of PickNow extracted from open source projects. You can rate examples to help us improve the quality of examples.
Note that I also used the Task<TResult> to initiate, so that the ContinueWith is strongly typed to provide the Dictionary<string, string> i.Result. Also note that you'll want to revisit what I'm doing in the i.Result.Aggregate logic. You'll need to update that to be correct to your case.
These pre-designed formats and designs can be utilized for various personal and expert tasks, consisting of resumes, invitations, leaflets, newsletters, reports, presentations, and more, streamlining the material development procedure. System Text Json Convert String To Json
Task<string> ，而不是 string 。 这是因为 ReadAsStringAsync() 返回一个 Task<string> 。 要打印您期望的 json，您需要编写... Console.WriteLine(GetResponseString("someToken").Result. 或者，您可以将方法切换为仅返回 . string 而不是 Task<string> ，然后修改该方法.....
We iterate over the cells in the row, and for each cell, we add an entry to the dictionary using the column letter as the key and the cell value as the value. Finally, we append the dictionary to the data list. In this article, we have discussed how to convert string list dictionaries from Python Excel file reading bunch.
I want to expose a property of Dictionary<string, MyAbstractClass>. I tried to do it this way: private Dictionary<string, MyChildClass> _dictionary; public Dictionary<string, MyAbstractClass> { get { return _dictionary; } //error: no implicit conversion.
Concatenating strings from several rows using pandas groupby. For this purpose, we will use the df.groupby() method by passing the set of the columns whose value we have to concatenate and a new column name. Also, we will apply the lambda expression with the .join() method. Consider the following code statement to achieve this task: