Lernen Handling Timeouts in Async Code | Timeouts, Parallelism, and Error Handling

You often need to set a time limit on how long your program should wait for an async operation to complete. Imagine waiting for a web page to load—if it takes too long, you might give up and try later. In async code, you want a way to "give up" if an operation is taking too long. This prevents your application from hanging or becoming unresponsive when something goes wrong, such as a slow network request or an unresponsive external service.

Definition

A CancellationToken is a struct that is passed to async methods to signal that the operation should be cancelled. It allows cooperative cancellation between threads or tasks.
Task.WhenAny is a method that returns a task which completes when any of the supplied tasks completes. It is useful for implementing timeouts by racing an operation against a timeout task.

Program.cs


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using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            Task longRunningTask = SimulateLongOperationAsync();
            Task timeoutTask = Task.Delay(2000); // 2 seconds timeout

            Task finishedTask = await Task.WhenAny(longRunningTask, timeoutTask);

            if (finishedTask == timeoutTask)
            {
                Console.WriteLine("Operation timed out.");
            }
            else
            {
                Console.WriteLine("Operation completed successfully.");
            }
        }

        public static async Task SimulateLongOperationAsync()
        {
            await Task.Delay(5000); // Simulate a long task (5 seconds)
        }
    }
}

To handle timeouts more flexibly, you can use a CancellationToken. A CancellationToken lets you signal to an async operation that it should stop what it is doing. This is especially useful when you want to cancel a task after a timeout or in response to user input. You create a CancellationTokenSource, start your async operation with its token, and then call Cancel() on the source if you need to stop the operation.

Example.cs


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// This is a demonstration of passing a CancellationToken to an async method.
// Not intended to be a runnable example.

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class Example
    {
        public async Task PerformOperationAsync(CancellationToken cancellationToken)
        {
            // Simulate work that checks for cancellation
            for (int i = 0; i < 10; i++)
            {
                cancellationToken.ThrowIfCancellationRequested();
                await Task.Delay(500, cancellationToken);
            }
        }

        public async Task StartWithTimeoutAsync()
        {
            using (var cts = new CancellationTokenSource(2000)) // 2 seconds timeout
            {
                try
                {
                    await PerformOperationAsync(cts.Token);
                    Console.WriteLine("Operation completed.");
                }
                catch (OperationCanceledException)
                {
                    Console.WriteLine("Operation was cancelled due to timeout.");
                }
            }
        }
    }
}

When implementing timeouts and cancellations, always handle them gracefully. Catch OperationCanceledException to clean up resources and inform users that the operation did not complete as expected. Avoid leaving tasks running in the background after you have given up on them. Make sure your async methods accept a CancellationToken and check it regularly, so they can respond quickly to cancellation requests. This approach leads to responsive, predictable, and user-friendly applications.

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