{"id":844,"date":"2023-03-10T11:33:41","date_gmt":"2023-03-10T04:33:41","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=844"},"modified":"2024-07-15T11:24:05","modified_gmt":"2024-07-15T04:24:05","slug":"csharp-whenall","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-concurrency\/csharp-whenall\/","title":{"rendered":"C# WhenAll"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the C# WhenAll()<\/code> static method of the Task class to create a task that will be completed when all the input tasks are completed.<\/p>\n\n\n\n

Introduction to the C# WhenAll() static method<\/h2>\n\n\n\n

The Task.WhenAll<\/code>()<\/code> method creates a task<\/a> that will be completed once all the input tasks are completed. The method returns a Task<\/code> object that represents the completion of all the input tasks. The returned task contains the results of all the input tasks.<\/p>\n\n\n\n

In practice, the Task.WhenAll<\/code>()<\/code> is useful for aggregating results from multiple asynchronous operations.<\/p>\n\n\n\n

The following program demonstrates the Task.WhenAll<\/code>()<\/code> method:<\/p>\n\n\n

static<\/span> async<\/span> Task<int<\/span>> DoWork<\/span>(int<\/span> taskId<\/span>)<\/span>\n{\n    Console.WriteLine($\"Task {taskId}<\/span> started...\"<\/span>);\n    await<\/span> Task.Delay(TimeSpan.FromSeconds(taskId));\n    Console.WriteLine($\"Task {taskId}<\/span> finished.\"<\/span>);\n    return<\/span> taskId * taskId;\n}\n\nvar<\/span> t1 = Task.Run(() => DoWork(1<\/span>));\nvar<\/span> t2 = Task.Run(() => DoWork(2<\/span>));\nvar<\/span> t3 = Task.Run(() => DoWork(3<\/span>));\n\nint<\/span>[] results = await<\/span> Task.WhenAll(t1, t2, t3);\n\nConsole.WriteLine(\"Results:\"<\/span>);\nConsole.WriteLine(string<\/span>.Join(\", \"<\/span>, results));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Task 3<\/span> started...\nTask 2<\/span> started...\nTask 1<\/span> started...\nTask 1<\/span> finished.\nTask 2<\/span> finished.\nTask 3<\/span> finished.\nResults:\n1<\/span>, 4<\/span>, 9<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define a method called DoWork()<\/code> that accepts an integer parameter taskId<\/code><\/code>. The method displays a message indicating that the task has started, and delays for a specified number of seconds using the Task.Delay<\/code>()<\/code> method, and displays another message showing that the task has finished and returns the square of the taskId<\/code><\/code>:<\/p>\n\n\n
static<\/span> async<\/span> Task<int<\/span>> DoWork<\/span>(int<\/span> taskId<\/span>)<\/span>\n{\n    Console.WriteLine($\"Task {taskId}<\/span> started...\"<\/span>);\n    await<\/span> Task.Delay(TimeSpan.FromSeconds(taskId));\n    Console.WriteLine($\"Task {taskId}<\/span> finished.\"<\/span>);\n    return<\/span> taskId * taskId;\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the method has an asynchronous operation, it is marked as an async<\/code> method. Hence, it returns a Task<int><\/code> object rather than an integer.<\/p>\n\n\n\n
Second, create three tasks using the Task.Run<\/code>()<\/code> method that executes the DoWork()<\/code> method on background threads:<\/p>\n\n\n
var<\/span> t1 = Task.Run(() => DoWork(1<\/span>));\nvar<\/span> t2 = Task.Run(() => DoWork(2<\/span>));\nvar<\/span> t3 = Task.Run(() => DoWork(3<\/span>));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, use the Task.WhenAll<\/code>()<\/code> method to wait for three tasks to be completed. The method returns an array of integers stored in the results variable:<\/p>\n\n\n
int<\/span>[] results = await<\/span> Task.WhenAll(t1, t2, t3);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, show the results to the console:<\/p>\n\n\n
Console.WriteLine(\"Results:\"<\/span>);\nConsole.WriteLine(string<\/span>.Join(\", \"<\/span>, results));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Waiting for multiple HTTP requests to complete using Task.WhenAll() method<\/h2>\n\n\n\n
The following example shows how to use the Task.WhenAll<\/code>()<\/code> and HttpClient<\/code> to fetch contents asynchronously from multiple URLs<\/code> and calculate the total bytes:<\/p>\n\n\n
static<\/span> async<\/span> Task<int<\/span>> Fetch<\/span>(string<\/span> url<\/span>)<\/span>\n{\n    Console.WriteLine($\"Fetching {url}<\/span>...\"<\/span>);\n\n    var<\/span> httpClient = new<\/span> HttpClient();\n    var<\/span> response = await<\/span> httpClient.GetAsync(url);\n    var<\/span> content = await<\/span> response.Content.ReadAsStringAsync();\n\n    Console.WriteLine($\"Fetched {url}<\/span> ({content.Length}<\/span> bytes)\"<\/span>);\n\n    return<\/span> content.Length;\n}\n\nvar<\/span> t1 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc2616\"<\/span>));\nvar<\/span> t2 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc2822\"<\/span>));\nvar<\/span> t3 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc1180\"<\/span>));\n\n\nvar<\/span> results = await<\/span> Task.WhenAll(new<\/span>[] { t1, t2, t3 });\n\nConsole.WriteLine($\"Total {results.Sum()}<\/span> bytes\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Fetching https:\/\/www.rfc-editor.org\/rfc\/rfc2822...<\/span>\nFetching https:\/\/www.rfc-editor.org\/rfc\/rfc1180...<\/span>\nFetching https:\/\/www.rfc-editor.org\/rfc\/rfc2616...<\/span>\nFetched https:\/\/www.rfc-editor.org\/rfc\/rfc2822 (141338 bytes)<\/span>\nFetched https:\/\/www.rfc-editor.org\/rfc\/rfc1180 (80606 bytes)<\/span>\nFetched https:\/\/www.rfc-editor.org\/rfc\/rfc2616 (521792 bytes)<\/span>\nTotal 743736<\/span> bytes<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define the Fetch()<\/code>method that takes a string argument url<\/code> and returns a Task<int><\/code> object:<\/p>\n\n\n
static<\/span> async<\/span> Task<int<\/span>> Fetch<\/span>(string<\/span> url<\/span>)<\/span>\n{\n    Console.WriteLine($\"Fetching {url}<\/span>...\"<\/span>);\n\n    var<\/span> httpClient = new<\/span> HttpClient();\n    var<\/span> response = await<\/span> httpClient.GetAsync(url);\n    var<\/span> content = await<\/span> response.Content.ReadAsStringAsync();\n\n    Console.WriteLine($\"Fetched {url}<\/span> ({content.Length}<\/span> bytes)\"<\/span>);\n\n    return<\/span> content.Length;\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The Fetch()<\/code> method uses the HttpClient<\/code> object to fetch the contents of the input url<\/code> asynchronously. Once it completes downloading the contents of the URL, it gets the size of the contents in bytes. and return it as the integer value wrapped in a Task<\/code> object.<\/p>\n\n\n\n
Second, create three tasks using the Task.Run<\/code> method to execute the Fetch()<\/code> method asynchronously:<\/p>\n\n\n
var<\/span> t1 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc2616\"<\/span>));\nvar<\/span> t2 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc2822\"<\/span>));\nvar<\/span> t3 = Task.Run(() => Fetch(\"https:\/\/www.rfc-editor.org\/rfc\/rfc1180\"<\/span>));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, wait for all three tasks to be completed using the Task.WhenAll<\/code>()<\/code> method and return an array of their results as Task<int[]><\/code> object:<\/p>\n\n\n
var<\/span> results = await<\/span> Task.WhenAll(new<\/span>[] { t1, t2, t3 });<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, calculate the total size of the fetched contents by adding up the size of the content of each task using the Sum()<\/code> extension method and display the result:<\/p>\n\n\n
Console.WriteLine($\"Total {results.Sum()}<\/span> bytes\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary<\/h2>\n\n\n\n
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  • Use the Task.WhenAll<\/code>()<\/code> method to create a task that will be completed once all the input tasks have finished.<\/li>\n<\/ul>\n
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