Impara Error Handling in Async Methods | Timeouts, Parallelism, and Error Handling

When working with asynchronous methods in C#, error handling behaves differently from what you might expect in synchronous code. In a synchronous method, exceptions are thrown immediately when an error occurs. However, in async methods, exceptions are captured and stored in the returned Task. The exception is not thrown at the point where the error happens, but rather when you await the task. This difference is crucial for writing correct and robust error handling logic in your async programs.

Program.cs


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using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            try
            {
                await PerformAsyncOperation();
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Caught exception in Main: {ex.Message}");
            }
        }

        public static async Task PerformAsyncOperation()
        {
            try
            {
                await Task.Delay(500);
                throw new InvalidOperationException("Something went wrong in async operation!");
            }
            catch (InvalidOperationException ex)
            {
                Console.WriteLine($"Handled in PerformAsyncOperation: {ex.Message}");
                throw; // Re-throw to propagate to caller
            }
        }
    }
}

When you await a task, any exception thrown inside the async method is re-thrown at the await point. This allows you to use familiar try/catch blocks around your await expressions. If you do not await the task, but instead access the Task.Exception property, you will find the exception wrapped inside an AggregateException. This is especially important when dealing with multiple tasks running in parallel. If you use Task.WhenAll to await several tasks at once, and more than one fails, all exceptions are collected and thrown together as an AggregateException. You must inspect the InnerExceptions property to see all the individual errors.

LoggingAndRecovery.cs


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using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    public class LoggingAndRecovery
    {
        public async Task RunAsyncTasksWithLogging()
        {
            Task task1 = Task.Run(() => throw new InvalidOperationException("Task 1 failed"));
            Task task2 = Task.Run(() => throw new ArgumentException("Task 2 failed"));

            try
            {
                await Task.WhenAll(task1, task2);
            }
            catch (AggregateException ex)
            {
                foreach (var inner in ex.InnerExceptions)
                {
                    // Log the error details
                    Console.WriteLine($"Error: {inner.Message}");
                }
                // Attempt recovery or fallback logic here
                Console.WriteLine("Attempting recovery after errors.");
            }
        }
    }
}

In real-world applications, you should always log exceptions that occur in async code, especially when running multiple tasks in parallel. Use try/catch blocks around your awaited tasks, and always check for AggregateException when using Task.WhenAll or similar APIs. For critical operations, consider implementing retry logic, fallback strategies, or user notifications when errors are encountered. Robust error handling in async code is essential for building reliable, fault-tolerant applications.

Definition

AggregateException is a special exception type in C# that represents one or more errors that occur during the execution of multiple tasks. When multiple tasks fail (such as with Task.WhenAll), their exceptions are combined into a single AggregateException, which contains a collection of all the individual exceptions in its InnerExceptions property.

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Sezione 3. Capitolo 5

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