Concurrency in C# Cookbook

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Concurrency in C# Cookbook

By: Stephen Cleary

Publisher: O'Reilly Media, Inc.

Pub. Date: June 3, 2014

Print ISBN-13: 978-1-4493-6756-5

Pages in Print Edition: 208

Subscriber Rating: [0 Ratings]

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关键词：currency Cookbook Cook Book curr Stephen Media

2.1. Pausing for a Period of Time
Problem
You need to (asynchronously) wait for a period of time. This can be useful when unit testing or implementing retry delays. This solution can also be useful for simple timeouts.
Solution
The Task type has a static method Delay that returns a task that completes after the specified time.

1. static async Task<string> DownloadStringWithRetries(string uri)
   
2. {
   
3.     using (var client = new HttpClient())
   
4.     {
   
5.         // Retry after 1 second, then after 2 seconds, then 4.
   
6.         var nextDelay = TimeSpan.FromSeconds(1);
   
7.         for (int i = 0; i != 3; ++i)
   
8.         {
   
9.             try
   
10.             {
    
11.                 return await client.GetStringAsync(uri);
    
12.             }
    
13.             catch
    
14.             {
    
15.             }
    
16. 
    
17.             await Task.Delay(nextDelay);
    
18.             nextDelay = nextDelay + nextDelay;
    
19.         }
    
20. 
    
21.         // Try one last time, allowing the error to propogate.
    
22.         return await client.GetStringAsync(uri);
    
23.     }
    
24. }

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1. static async Task<string> DownloadStringWithTimeout(string uri)
   
2. {
   
3.     using (var client = new HttpClient())
   
4.     {
   
5.         var downloadTask = client.GetStringAsync(uri);
   
6.         var timeoutTask = Task.Delay(3000);
   
7. 
   
8.         var completedTask = await Task.WhenAny(downloadTask, timeoutTask);
   
9.         if (completedTask == timeoutTask)
   
10.             return null;
    
11.         return await downloadTask;
    
12.     }
    
13. }

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2.2. Returning Completed Tasks
Problem
You need to implement a synchronous method with an asynchronous signature. This situation can arise if you are inheriting from an asynchronous interface or base class but wish to implement it synchronously. This technique is particularly useful when unit testing asynchronous code, when you need a simple stub or mock for an asynchronous interface.
Solution
You can use Task.FromResult to create and return a new Task<T> that is already completed with the specified value:

1. interface IMyAsyncInterface
   
2. {
   
3.     Task<int> GetValueAsync();
   
4. }
   
5. 
   
6. class MySynchronousImplementation : IMyAsyncInterface
   
7. {
   
8.     public Task<int> GetValueAsync()
   
9.     {
   
10.         return Task.FromResult(13);
    
11.     }
    
12. }

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2.3. Reporting Progress
Problem
You need to respond to progress while an asynchronous operation is executing.
Solution
Use the provided IProgress<T> and Progress<T> types. Your async method should take an IProgress<T> argument; the T is whatever type of progress you need to report:

1. static async Task MyMethodAsync(IProgress<double> progress = null)
   
2. {
   
3.     double percentComplete = 0;
   
4.     while (!done)
   
5.     {
   
6.         ...
   
7.         if (progress != null)
   
8.             progress.Report(percentComplete);
   
9.     }
   
10. }
    
11. Calling code can use it as such:
    
12. static async Task CallMyMethodAsync()
    
13. {
    
14.     var progress = new Progress<double>();
    
15.     progress.ProgressChanged += (sender, args) =>
    
16.     {
    
17.         ...
    
18.     };
    
19.     await MyMethodAsync(progress);
    
20. }

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2.3. Reporting Progress
Problem
You need to respond to progress while an asynchronous operation is executing.
Solution
Use the provided IProgress<T> and Progress<T> types. Your async method should take an IProgress<T> argument; the T is whatever type of progress you need to report:

1. static async Task MyMethodAsync(IProgress<double> progress = null)
   
2. {
   
3.     double percentComplete = 0;
   
4.     while (!done)
   
5.     {
   
6.         ...
   
7.         if (progress != null)
   
8.             progress.Report(percentComplete);
   
9.     }
   
10. }
    
11. Calling code can use it as such:
    
12. static async Task CallMyMethodAsync()
    
13. {
    
14.     var progress = new Progress<double>();
    
15.     progress.ProgressChanged += (sender, args) =>
    
16.     {
    
17.         ...
    
18.     };
    
19.     await MyMethodAsync(progress);
    
20. }

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