C# Async Patterns

Asynchronous programming in C# enables writing responsive applications that efficiently handle I/O-bound and CPU-bound operations without blocking threads. The async/await pattern provides a straightforward way to write asynchronous code that looks and behaves like synchronous code.

Async/Await Basics

The async and await keywords transform synchronous-looking code into state machines that handle asynchronous operations.

using System;
using System.Net.Http;
using System.Threading.Tasks;

public class AsyncBasics
{
    // Basic async method
    public async Task<string> FetchDataAsync(string url)
    {
        using var client = new HttpClient();
        // await suspends execution until response received
        string content = await client.GetStringAsync(url);
        return content;
    }

    // Async method without return value
    public async Task ProcessDataAsync()
    {
        await Task.Delay(1000); // Simulate async work
        Console.WriteLine("Processing complete");
    }

    // Multiple awaits
    public async Task<int> CalculateSumAsync()
    {
        int value1 = await GetValueAsync(1);
        int value2 = await GetValueAsync(2);
        int value3 = await GetValueAsync(3);

        return value1 + value2 + value3;
    }

    private async Task<int> GetValueAsync(int id)
    {
        await Task.Delay(100);
        return id * 10;
    }

    // Async with exception handling
    public async Task<string> SafeFetchAsync(string url)
    {
        try
        {
            using var client = new HttpClient();
            return await client.GetStringAsync(url);
        }
        catch (HttpRequestException ex)
        {
            Console.WriteLine($"Request failed: {ex.Message}");
            return string.Empty;
        }
    }

    // Calling async methods
    public async Task DemoAsync()
    {
        // Await the result
        string data = await FetchDataAsync("https://api.example.com");

        // Fire and forget (not recommended)
        _ = ProcessDataAsync();

        // Wait for completion
        await ProcessDataAsync();
    }
}

Task and Task<T>

Task represents an asynchronous operation and provides methods for composition, continuation, and error handling.

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

public class TaskExamples
{
    // Creating tasks
    public void CreateTasks()
    {
        // Task.Run for CPU-bound work
        Task<int> task1 = Task.Run(() =>
        {
            Thread.Sleep(1000);
            return 42;
        });

        // Task.FromResult for already-known values
        Task<int> task2 = Task.FromResult(100);

        // Task.CompletedTask for void operations
        Task task3 = Task.CompletedTask;

        // TaskCompletionSource for manual control
        var tcs = new TaskCompletionSource<string>();
        Task<string> task4 = tcs.Task;
        tcs.SetResult("Done");
    }

    // Task composition
    public async Task<string> ComposeTasks()
    {
        // Sequential execution
        int result1 = await Task1Async();
        int result2 = await Task2Async(result1);

        // Parallel execution
        Task<int> t1 = Task1Async();
        Task<int> t2 = Task2Async(10);
        await Task.WhenAll(t1, t2);

        return $"Results: {t1.Result}, {t2.Result}";
    }

    // Task.WhenAll - wait for all tasks
    public async Task<int[]> WhenAllExample()
    {
        var tasks = new[]
        {
            Task.Run(() => ComputeValue(1)),
            Task.Run(() => ComputeValue(2)),
            Task.Run(() => ComputeValue(3))
        };

        int[] results = await Task.WhenAll(tasks);
        return results;
    }

    // Task.WhenAny - wait for first task
    public async Task<int> WhenAnyExample()
    {
        var task1 = DelayedValue(1000, 1);
        var task2 = DelayedValue(2000, 2);
        var task3 = DelayedValue(500, 3);

        Task<int> completed = await Task.WhenAny(task1, task2, task3);
        return await completed;
    }

    // Cancellation support
    public async Task<string> CancellableOperation(
        CancellationToken cancellationToken)
    {
        for (int i = 0; i < 10; i++)
        {
            cancellationToken.ThrowIfCancellationRequested();

            await Task.Delay(100, cancellationToken);
            Console.WriteLine($"Step {i + 1}");
        }

        return "Completed";
    }

    // Helper methods
    private Task<int> Task1Async() => Task.FromResult(10);
    private Task<int> Task2Async(int value) =>
        Task.FromResult(value * 2);
    private int ComputeValue(int x) => x * x;
    private async Task<int> DelayedValue(int delay, int value)
    {
        await Task.Delay(delay);
        return value;
    }
}

ValueTask and ValueTask<T>

ValueTask provides better performance for operations that often complete synchronously, avoiding heap allocations.

using System;
using System.Threading.Tasks;

public class ValueTaskExamples
{
    private readonly Dictionary<string, string> _cache =
        new Dictionary<string, string>();

    // ValueTask for cached operations
    public ValueTask<string> GetValueAsync(string key)
    {
        // Synchronous path - no allocation
        if (_cache.TryGetValue(key, out string? value))
        {
            return new ValueTask<string>(value);
        }

        // Asynchronous path
        return new ValueTask<string>(FetchFromDatabaseAsync(key));
    }

    private async Task<string> FetchFromDatabaseAsync(string key)
    {
        await Task.Delay(100); // Simulate database call
        string value = $"Value for {key}";
        _cache[key] = value;
        return value;
    }

    // Converting between Task and ValueTask
    public async ValueTask<int> ConversionExample()
    {
        // ValueTask from Task
        Task<int> task = GetTaskAsync();
        ValueTask<int> valueTask = new ValueTask<int>(task);

        return await valueTask;
    }

    private Task<int> GetTaskAsync() => Task.FromResult(42);

    // ValueTask best practices
    public async Task ValueTaskUsageAsync()
    {
        // Good: await immediately
        string value1 = await GetValueAsync("key1");

        // Bad: storing ValueTask
        // ValueTask<string> vt = GetValueAsync("key2");
        // await vt; // First await
        // await vt; // Second await - WRONG!

        // Good: convert to Task if needed multiple times
        Task<string> task = GetValueAsync("key2").AsTask();
        await task;
        await task; // OK with Task
    }

    // ConfigureAwait with ValueTask
    public async ValueTask ConfigureAwaitExample()
    {
        // Don't capture context (for library code)
        string value = await GetValueAsync("key")
            .ConfigureAwait(false);

        Console.WriteLine(value);
    }
}

ConfigureAwait

ConfigureAwait controls whether to capture the synchronization context, critical for library code and avoiding deadlocks.

using System;
using System.Threading.Tasks;

public class ConfigureAwaitExamples
{
    // Library method - use ConfigureAwait(false)
    public async Task<string> LibraryMethodAsync()
    {
        // Don't capture synchronization context
        await Task.Delay(100).ConfigureAwait(false);

        // This continues on thread pool thread
        string result = await GetDataAsync()
            .ConfigureAwait(false);

        return result.ToUpper();
    }

    // UI method - use default (or ConfigureAwait(true))
    public async Task UpdateUIAsync()
    {
        string data = await LoadDataAsync();

        // This continues on UI thread
        // Can safely update UI controls
        Console.WriteLine($"Data: {data}");
    }

    // Avoiding deadlocks
    public class DeadlockExample
    {
        // This can deadlock in synchronous context
        public string BadSync()
        {
            // DON'T DO THIS
            return GetDataAsync().Result; // Deadlock!
        }

        // Fix with ConfigureAwait(false)
        public string GoodSync()
        {
            return GetDataAsync()
                .ConfigureAwait(false)
                .GetAwaiter()
                .GetResult();
        }

        // Better: make it async
        public async Task<string> BestAsync()
        {
            return await GetDataAsync();
        }
    }

    // Mixing contexts
    public async Task MixedContextAsync()
    {
        // Runs on captured context
        await Task.Delay(100);
        Console.WriteLine("On original context");

        // Runs on thread pool
        await Task.Delay(100).ConfigureAwait(false);
        Console.WriteLine("On thread pool");

        // Still on thread pool (ConfigureAwait effect persists)
        await Task.Delay(100);
        Console.WriteLine("Still on thread pool");
    }

    private async Task<string> GetDataAsync()
    {
        await Task.Delay(100);
        return "data";
    }

    private async Task<string> LoadDataAsync()
    {
        await Task.Delay(100);
        return "loaded data";
    }
}

Async Streams (IAsyncEnumerable)

Async streams enable asynchronous iteration over sequences of data, perfect for streaming APIs and large datasets.

using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;

public class AsyncStreamExamples
{
    // Basic async stream
    public async IAsyncEnumerable<int> GenerateNumbersAsync(
        int count)
    {
        for (int i = 0; i < count; i++)
        {
            await Task.Delay(100);
            yield return i;
        }
    }

    // Consuming async stream
    public async Task ConsumeStreamAsync()
    {
        await foreach (int number in GenerateNumbersAsync(10))
        {
            Console.WriteLine(number);
        }
    }

    // Async stream with cancellation
    public async IAsyncEnumerable<string> ReadLinesAsync(
        string filePath,
        [EnumeratorCancellation] CancellationToken cancellationToken =
            default)
    {
        using var reader = new System.IO.StreamReader(filePath);

        while (!reader.EndOfStream)
        {
            cancellationToken.ThrowIfCancellationRequested();

            string? line = await reader.ReadLineAsync();
            if (line != null)
            {
                yield return line;
            }
        }
    }

    // Filtering async stream
    public async IAsyncEnumerable<int> FilterEvenNumbersAsync(
        IAsyncEnumerable<int> source)
    {
        await foreach (int number in source)
        {
            if (number % 2 == 0)
            {
                yield return number;
            }
        }
    }

    // Transforming async stream
    public async IAsyncEnumerable<string> FormatNumbersAsync(
        IAsyncEnumerable<int> source)
    {
        await foreach (int number in source)
        {
            yield return $"Number: {number:D3}";
        }
    }

    // Async stream from API
    public async IAsyncEnumerable<string> FetchPagesAsync(
        string baseUrl,
        int totalPages)
    {
        using var client = new HttpClient();

        for (int page = 1; page <= totalPages; page++)
        {
            string url = $"{baseUrl}?page={page}";
            string content = await client.GetStringAsync(url);
            yield return content;
        }
    }

    // Composing async streams
    public async Task ComposeStreamsAsync()
    {
        var numbers = GenerateNumbersAsync(20);
        var evens = FilterEvenNumbersAsync(numbers);
        var formatted = FormatNumbersAsync(evens);

        await foreach (string value in formatted)
        {
            Console.WriteLine(value);
        }
    }
}

Parallel Async Operations

Combining parallelism with async operations for maximum throughput.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

public class ParallelAsyncExamples
{
    // Process items in parallel
    public async Task<List<string>> ProcessInParallelAsync(
        List<int> items)
    {
        var tasks = items.Select(async item =>
        {
            await Task.Delay(100);
            return $"Processed {item}";
        });

        string[] results = await Task.WhenAll(tasks);
        return results.ToList();
    }

    // Throttled parallel execution
    public async Task<List<string>> ThrottledParallelAsync(
        List<int> items,
        int maxConcurrency)
    {
        var semaphore = new SemaphoreSlim(maxConcurrency);
        var tasks = items.Select(async item =>
        {
            await semaphore.WaitAsync();
            try
            {
                await Task.Delay(100);
                return $"Processed {item}";
            }
            finally
            {
                semaphore.Release();
            }
        });

        string[] results = await Task.WhenAll(tasks);
        return results.ToList();
    }

    // Parallel.ForEachAsync (.NET 6+)
    public async Task ParallelForEachAsyncExample(List<int> items)
    {
        await Parallel.ForEachAsync(
            items,
            new ParallelOptions { MaxDegreeOfParallelism = 4 },
            async (item, cancellationToken) =>
            {
                await Task.Delay(100, cancellationToken);
                Console.WriteLine($"Processed {item}");
            });
    }

    // Batched processing
    public async Task<List<string>> BatchProcessAsync(
        List<int> items,
        int batchSize)
    {
        var results = new List<string>();

        for (int i = 0; i < items.Count; i += batchSize)
        {
            var batch = items.Skip(i).Take(batchSize);
            var batchResults = await ProcessInParallelAsync(
                batch.ToList());
            results.AddRange(batchResults);
        }

        return results;
    }
}

Error Handling in Async Code

Proper error handling is crucial for robust asynchronous applications.

using System;
using System.Threading.Tasks;

public class AsyncErrorHandling
{
    // Basic try-catch
    public async Task<string> BasicErrorHandlingAsync()
    {
        try
        {
            return await RiskyOperationAsync();
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine($"Operation error: {ex.Message}");
            return "default";
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Unexpected error: {ex.Message}");
            throw;
        }
    }

    // AggregateException from WhenAll
    public async Task HandleMultipleErrorsAsync()
    {
        try
        {
            await Task.WhenAll(
                FailingTaskAsync("Task 1"),
                FailingTaskAsync("Task 2"),
                FailingTaskAsync("Task 3")
            );
        }
        catch (Exception ex)
        {
            // Only first exception caught
            Console.WriteLine($"First error: {ex.Message}");
        }
    }

    // Handling all exceptions
    public async Task HandleAllErrorsAsync()
    {
        var tasks = new[]
        {
            FailingTaskAsync("Task 1"),
            FailingTaskAsync("Task 2"),
            FailingTaskAsync("Task 3")
        };

        try
        {
            await Task.WhenAll(tasks);
        }
        catch
        {
            // Iterate through all tasks to see all exceptions
            foreach (var task in tasks)
            {
                if (task.IsFaulted && task.Exception != null)
                {
                    foreach (var ex in task.Exception.InnerExceptions)
                    {
                        Console.WriteLine($"Error: {ex.Message}");
                    }
                }
            }
        }
    }

    // finally blocks work as expected
    public async Task FinallyBlockAsync()
    {
        try
        {
            await RiskyOperationAsync();
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
        finally
        {
            // Always executes
            Console.WriteLine("Cleanup code");
        }
    }

    private async Task<string> RiskyOperationAsync()
    {
        await Task.Delay(100);
        throw new InvalidOperationException("Something went wrong");
    }

    private async Task FailingTaskAsync(string name)
    {
        await Task.Delay(100);
        throw new InvalidOperationException($"{name} failed");
    }
}

Best Practices

1. Use async/await all the way - avoid mixing async and sync code
2. Prefer Task.Run for CPU-bound work, native async APIs for I/O
3. Use ValueTask<T> for hot paths that often complete synchronously
4. Always use ConfigureAwait(false) in library code
5. Never use .Result or .Wait() on Tasks - causes deadlocks
6. Properly handle cancellation with CancellationToken
7. Use Task.WhenAll for parallel operations, not sequential awaits
8. Implement proper exception handling for async operations
9. Use async streams for sequences that are produced asynchronously
10. Avoid async void except for event handlers

Common Pitfalls

1. Blocking on async code with .Result or .Wait() causing deadlocks
2. Not using ConfigureAwait(false) in library code capturing context unnecessarily
3. Using async void methods which can't be properly awaited or caught
4. Forgetting to await tasks, causing fire-and-forget behavior
5. Not handling exceptions in parallel tasks properly
6. Over-parallelizing with too many concurrent operations
7. Using Task.Run for already-async I/O operations (double wrapping)
8. Not passing CancellationToken through async call chains
9. Storing and awaiting ValueTask multiple times
10. Capturing large objects in async lambda closures causing memory issues

When to Use Async Patterns

Use async patterns when you need:

• Responsive UI applications that don't freeze during I/O operations
• Web APIs and services handling many concurrent requests efficiently
• Database operations that shouldn't block threads
• File I/O operations for reading and writing large files
• Network operations including HTTP requests and socket communication
• Streaming large datasets without loading everything into memory
• CPU-bound work offloaded to thread pool with Task.Run
• Composable asynchronous operations with proper error handling
• Cancellable long-running operations
• Maximum scalability in server applications

Resources

• Async/Await Best Practices
• ValueTask Documentation
• Asynchronous Programming Patterns
• Async Streams Tutorial