{"id":912,"date":"2023-03-12T09:55:46","date_gmt":"2023-03-12T02:55:46","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=912"},"modified":"2023-03-12T09:57:02","modified_gmt":"2023-03-12T02:57:02","slug":"csharp-countdownevent","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-concurrency\/csharp-countdownevent\/","title":{"rendered":"C# CountdownEvent"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the C# CountdownEvent<\/code> class to wait for a specified number of events before continuing execution.<\/p>\n\n\n\n

Introduction to the C# CountdownEvent class<\/h2>\n\n\n\n

The CountdownEvent<\/code><\/code> unblocks a waiting thread<\/a> until it receives the number of events. To use the CountdownEvent<\/code><\/code> class, you follow these steps:<\/p>\n\n\n\n

First, create a new instance of the CountdownEvent<\/code> class, passing an initial count value:<\/p>\n\n\n

var<\/span> countdownEvent = new<\/span> CountdownEvent(3<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create one or more threads that will signal an event. Each thread needs to call the Signal()<\/code> method of the CountdownEvent<\/code> object to signal an event:<\/p>\n\n\n
\/\/ inside a thread<\/span>\ncountdownEvent.Signal();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The Signal()<\/code> method decrements the initial count by one.<\/p>\n\n\n\n
Third, call the Wait()<\/code><\/code> method of the CountdownEvent<\/code> object on the waiting threads. The Wait()<\/code><\/code> method will block these threads until the count reaches zero:<\/p>\n\n\n
\/\/ on the waiting thread<\/span>\ncountdownEvent.Wait();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
If an exception occurs that causes the Signal()<\/code> method not to be called, the Wait()<\/code> method will block the waiting thread indefinitely. <\/p>\n\n\n\n
Therefore, you should always call the Signal()<\/code> method even if an exception occurs. <\/p>\n\n\n\n
Also, you can set a timeout so that the Wait()<\/code> method will unblock the waiting thread if a time interval has elapsed or the count of the CountDownEvent<\/code> object has reached zero:<\/p>\n\n\n
countdownEvent.Wait(timeout);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Finally, call the Dispose()<\/code> method once you’re done using the CountdownEvent<\/code> object to release any resources associated with it:<\/p>\n\n\n
countdownEvent.Dispose();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example demonstrates how the CountdownEvent<\/code> class works:<\/p>\n\n\n
using<\/span> static<\/span> System.Console;\n\nvar<\/span> countdownEvent = new<\/span> CountdownEvent(3<\/span>);\n\nvoid<\/span> DoWork<\/span>(int<\/span> id<\/span>)<\/span>\n{\n    Thread.Sleep(1000<\/span>);\n    WriteLine($\"Completed the task {id}<\/span> \"<\/span>);\n    countdownEvent.Signal();\n\n}\n\n\/\/ create three threads that executes the DoWork<\/span>\nfor<\/span> (int<\/span> i = 1<\/span>; i <= 3<\/span>; i++)\n{\n    var<\/span> id = i;\n    var<\/span> thread = new<\/span> Thread(() => DoWork(id));\n    thread.Start();\n}\n\n\n\/\/ block the main thread<\/span>\ncountdownEvent.Wait();\n\n\/\/ execute this once three event has been signaled<\/span>\nConsole.WriteLine(\"All thread completed.\"<\/span>);\n\ncountdownEvent.Dispose();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Completed the task 2<\/span>\nCompleted the task 1<\/span>\nCompleted the task 3<\/span>\nAll thread completed.<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, create a new CountdownEvent<\/code> object with the initial count 3:<\/p>\n\n\n
var<\/span> countdownEvent = new<\/span> CountdownEvent(3<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, define the DoWork()<\/code> method that simulates a task taking one second to complete:<\/p>\n\n\n
void<\/span> DoWork<\/span>(int<\/span> id<\/span>)<\/span>\n{\n    Thread.Sleep(1000<\/span>);\n    WriteLine($\"Completed the task {id}<\/span> \"<\/span>);\n    countdownEvent.Signal();\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
The DoWork()<\/code> calls the Signal()<\/code> method of the CountdownEvent<\/code> object to decrement the count once it completes. <\/p>\n\n\n\n
Third, create three threads that execute the DoWork()<\/code> method:<\/p>\n\n\n
for<\/span> (int<\/span> i = 1<\/span>; i <= 3<\/span>; i++)\n{\n    var<\/span> id = i;\n    var<\/span> thread = new<\/span> Thread(() => DoWork(id));\n    thread.Start();\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Fourth, call the Wait()<\/code> method on the main thread to block it until the CountdownEvent<\/code> object receives three signals:<\/p>\n\n\n
countdownEvent.Wait();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Once the CountdownEvent<\/code><\/code> reaches zero, the main thread is unblocked that executes the rest of the code, writing a message to the console and disposing of the CountdownEvent<\/code><\/code> object:<\/p>\n\n\n
\/\/ execute this once three event has been signaled<\/span>\nConsole.WriteLine(\"All thread completed.\"<\/span>);\n\ncountdownEvent.Dispose();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
C# CountdownEvent applications<\/h2>\n\n\n\n
In practice, you can use the CountdownEvent<\/code> class in the following applications:<\/p>\n\n\n\n
    \n
  • Parallel processing:  you can process a large dataset in multiple threads and use a CountdownEvent<\/code> object to synchronize the completion of all threads before proceeding to the next step in the processing pipeline.<\/li>\n\n\n\n
  • File download: you can download multiple files in parallel and use a CountdownEvent<\/code> object to wait until all the files have been downloaded before processing them.<\/li>\n\n\n\n
  • Waiting for external events: suppose your program needs to wait for multiple external events such as network messages or sensor readings before proceeding. In this case, you can use a CountdownEvent<\/code> object to ensure all signals have been received before continuing.<\/li>\n<\/ul>\n\n\n\n
    A practical example of C# CountdownEvent class<\/h2>\n\n\n\n
    The following program downloads three text files concurrently from a remote server and counts the words of each. It uses the CountdownEvent<\/code> object to wait for the downloads to complete before returning the total word count of three files:<\/p>\n\n\n
    \nvar<\/span> urls = new<\/span> List<string<\/span>>()\n{\n    \"https:\/\/www.ietf.org\/rfc\/rfc791.txt\"<\/span>,\n    \"https:\/\/www.ietf.org\/rfc\/rfc792.txt\"<\/span>,\n    \"https:\/\/www.ietf.org\/rfc\/rfc793.txt\"<\/span>\n};\n\nvar<\/span> countDownEvent = new<\/span> CountdownEvent(urls.Count);\nint<\/span> totalWordCount = 0<\/span>;\n\nasync<\/span> Task<int<\/span>> DownloadAndCountWords<\/span>(string<\/span> url<\/span>)<\/span>\n{\n    Console.WriteLine($\"Downloading the file {url}<\/span>...\"<\/span>);\n\n    using<\/span> var<\/span> client = new<\/span> HttpClient();\n    string<\/span> content = await<\/span> client.GetStringAsync(url);\n\n    Console.WriteLine($\"File {url}<\/span> downloaded.\"<\/span>);\n\n    int<\/span> wordCount = CountWords(content);\n    Console.WriteLine($\"Word count of {url}<\/span>: {wordCount}<\/span>\"<\/span>);\n\n    return<\/span> wordCount;\n}\n\nint<\/span> CountWords<\/span>(string<\/span> text<\/span>)<\/span>\n{\n    var<\/span> delimiters = new<\/span> char<\/span>[] { ' '<\/span>, '\\r'<\/span>, '\\n'<\/span> };\n    return<\/span> text.Split(delimiters, StringSplitOptions.RemoveEmptyEntries).Length;\n}\n\n\nurls.ForEach(url =>\n{\n    Task.Run(async<\/span> () =>\n    {\n        int<\/span> wordCount = await<\/span> DownloadAndCountWords(url);\n        Interlocked.Add(ref<\/span> totalWordCount, wordCount);\n        \n        \/\/ signal an event<\/span>\n        countDownEvent.Signal();\n    });\n\n});\n\ncountDownEvent.Wait(TimeSpan.FromSeconds(3<\/span>));\ncountDownEvent.Dispose();\n\nConsole.WriteLine($\"Total word count: {totalWordCount}<\/span>\"<\/span>);\nConsole.WriteLine(\"Press any key to exit.\"<\/span>);\nConsole.ReadKey();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Output:<\/p>\n\n\n
    Downloading the file https:\/\/www.ietf.org\/rfc\/rfc791.txt...<\/span>\nDownloading the file https:\/\/www.ietf.org\/rfc\/rfc792.txt...<\/span>\nDownloading the file https:\/\/www.ietf.org\/rfc\/rfc793.txt...<\/span>\nFile https:\/\/www.ietf.org\/rfc\/rfc792.txt downloaded.<\/span>\nWord count of https:\/\/www.ietf.org\/rfc\/rfc792.txt: 3714<\/span>\nFile https:\/\/www.ietf.org\/rfc\/rfc791.txt downloaded.<\/span>\nWord count of https:\/\/www.ietf.org\/rfc\/rfc791.txt: 11243<\/span>\nFile https:\/\/www.ietf.org\/rfc\/rfc793.txt downloaded.<\/span>\nWord count of https:\/\/www.ietf.org\/rfc\/rfc793.txt: 21460<\/span>\nTotal word count: 36417<\/span>\nPress any key to exit.<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
    Summary<\/h2>\n\n\n\n
      \n
    • Use C# CountdownEvent<\/code> to allow threads to wait until a specified number of events have been signaled.<\/li>\n<\/ul>\n
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