C# Async Patterns

Master asynchronous programming in C# using async/await, Task, ValueTask, async streams, and cancellation patterns. This skill covers modern asynchronous patterns from C# 8-12 for building responsive, scalable applications.

Async/Await Fundamentals

The async/await pattern provides a simple way to write asynchronous code that looks and behaves like synchronous code.

Basic Async Method

public async Task<string> FetchDataAsync(string url)
{
    using var client = new HttpClient();
    string result = await client.GetStringAsync(url);
    return result;
}

// Calling the async method
public async Task ProcessAsync()
{
    string data = await FetchDataAsync("https://api.example.com/data");
    Console.WriteLine(data);
}

Async Method Signature Rules

// ✅ Correct - Returns Task
public async Task ProcessDataAsync()
{
    await Task.Delay(1000);
}

// ✅ Correct - Returns Task<T>
public async Task<int> CalculateAsync()
{
    await Task.Delay(1000);
    return 42;
}

// ⚠️ Only for event handlers - Returns void
public async void Button_Click(object sender, EventArgs e)
{
    await ProcessDataAsync();
}

// ❌ Wrong - Not async but returns Task
public Task WrongAsync()
{
    // Should be async or use Task.FromResult
    return Task.CompletedTask;
}

Task and Task<T>

Task represents an asynchronous operation. Task<T> represents an operation that returns a value.

Creating Tasks

// Task.Run for CPU-bound work
public async Task<int> CalculateSumAsync(int[] numbers)
{
    return await Task.Run(() => numbers.Sum());
}

// Task.FromResult for already-computed values
public Task<string> GetCachedValueAsync(string key)
{
    if (_cache.TryGetValue(key, out var value))
    {
        return Task.FromResult(value);
    }
    return FetchFromDatabaseAsync(key);
}

// Task.CompletedTask for void async methods
public Task ProcessIfNeededAsync(bool condition)
{
    if (!condition)
    {
        return Task.CompletedTask;
    }
    return DoActualWorkAsync();
}

Task Composition

public async Task<Result> ProcessOrderAsync(Order order)
{
    // Sequential execution
    await ValidateOrderAsync(order);
    await ChargePaymentAsync(order);
    await ShipOrderAsync(order);

    return new Result { Success = true };
}

public async Task<Result> ProcessOrderParallelAsync(Order order)
{
    // Parallel execution
    var validationTask = ValidateOrderAsync(order);
    var inventoryTask = CheckInventoryAsync(order);
    var pricingTask = CalculatePricingAsync(order);

    await Task.WhenAll(validationTask, inventoryTask, pricingTask);

    return new Result
    {
        IsValid = await validationTask,
        InStock = await inventoryTask,
        Price = await pricingTask
    };
}

ValueTask and ValueTask<T>

ValueTask is a performance optimization for scenarios where the result is often available synchronously.

When to Use ValueTask

public class CachedRepository
{
    private readonly Dictionary<int, User> _cache = new();
    private readonly IDatabase _database;

    // ✅ Good use of ValueTask - often returns synchronously
    // from cache
    public ValueTask<User> GetUserAsync(int id)
    {
        if (_cache.TryGetValue(id, out var user))
        {
            return ValueTask.FromResult(user);
        }

        return new ValueTask<User>(FetchUserFromDatabaseAsync(id));
    }

    private async Task<User> FetchUserFromDatabaseAsync(int id)
    {
        var user = await _database.QueryAsync<User>(id);
        _cache[id] = user;
        return user;
    }
}

ValueTask Best Practices

public class BufferedReader
{
    private readonly byte[] _buffer = new byte[4096];
    private int _position;
    private int _length;

    // ValueTask for hot path optimization
    public async ValueTask<byte> ReadByteAsync()
    {
        if (_position < _length)
        {
            // Synchronous path - no allocation
            return _buffer[_position++];
        }

        // Asynchronous path - read more data
        await FillBufferAsync();
        return _buffer[_position++];
    }

    private async Task FillBufferAsync()
    {
        _length = await _stream.ReadAsync(_buffer);
        _position = 0;
    }
}

// ⚠️ ValueTask rules
public async Task ConsumeValueTaskAsync()
{
    var reader = new BufferedReader();

    // ✅ Correct - await once
    byte b = await reader.ReadByteAsync();

    // ❌ Wrong - don't store ValueTask
    var task = reader.ReadByteAsync();
    await task; // Potential issues

    // ❌ Wrong - don't await multiple times
    var vt = reader.ReadByteAsync();
    await vt;
    await vt; // NEVER do this
}

Async Void vs Async Task

Understanding when to use async void (rarely) versus async Task (almost always).

The Async Void Problem

// ❌ Bad - Cannot await, exceptions unhandled
public async void ProcessDataBadAsync()
{
    await Task.Delay(1000);
    throw new Exception("Unhandled!"); // Crashes app
}

// ✅ Good - Can await, exceptions handled
public async Task ProcessDataGoodAsync()
{
    await Task.Delay(1000);
    throw new Exception("Handled!"); // Can be caught
}

// Usage
public async Task CallerAsync()
{
    try
    {
        // Cannot await async void
        ProcessDataBadAsync(); // Fire and forget - DANGEROUS

        // Can await async Task
        await ProcessDataGoodAsync(); // Exception caught here
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Caught: {ex.Message}");
    }
}

The Only Valid Use of Async Void

// ✅ Event handlers - the ONLY valid use case
public partial class MainWindow : Window
{
    public async void SaveButton_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            await SaveDataAsync();
            MessageBox.Show("Saved successfully!");
        }
        catch (Exception ex)
        {
            MessageBox.Show($"Error: {ex.Message}");
        }
    }

    private async Task SaveDataAsync()
    {
        await _repository.SaveAsync(_data);
    }
}

ConfigureAwait(false)

Control synchronization context capture for performance in library code.

Understanding ConfigureAwait

// Library code - use ConfigureAwait(false)
public class DataService
{
    public async Task<Data> GetDataAsync(int id)
    {
        // ConfigureAwait(false) - don't capture context
        var json = await _httpClient.GetStringAsync($"/api/data/{id}")
            .ConfigureAwait(false);

        var data = await DeserializeAsync(json)
            .ConfigureAwait(false);

        return data;
    }
}

// UI code - DON'T use ConfigureAwait(false)
public class ViewModel
{
    public async Task LoadDataAsync()
    {
        var data = await _dataService.GetDataAsync(42);
        // Need UI context here
        this.DataProperty = data; // Update UI
    }
}

ConfigureAwait Patterns

public class AsyncLibrary
{
    // ✅ Library method with ConfigureAwait(false)
    public async Task<Result> ProcessAsync(string input)
    {
        var step1 = await Step1Async(input).ConfigureAwait(false);
        var step2 = await Step2Async(step1).ConfigureAwait(false);
        var step3 = await Step3Async(step2).ConfigureAwait(false);
        return step3;
    }

    // ✅ ASP.NET Core - ConfigureAwait(false) safe everywhere
    [HttpGet]
    public async Task<IActionResult> GetData(int id)
    {
        // ASP.NET Core has no synchronization context
        var data = await _repository.GetAsync(id).ConfigureAwait(false);
        return Ok(data);
    }
}

CancellationToken Patterns

Proper cancellation support for long-running operations.

Basic Cancellation

public async Task<List<Result>> ProcessItemsAsync(
    IEnumerable<Item> items,
    CancellationToken cancellationToken = default)
{
    var results = new List<Result>();

    foreach (var item in items)
    {
        // Check for cancellation
        cancellationToken.ThrowIfCancellationRequested();

        var result = await ProcessItemAsync(item, cancellationToken);
        results.Add(result);
    }

    return results;
}

// Usage with timeout
public async Task<List<Result>> ProcessWithTimeoutAsync(IEnumerable<Item> items)
{
    using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));

    try
    {
        return await ProcessItemsAsync(items, cts.Token);
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine("Operation timed out");
        throw;
    }
}

Advanced Cancellation Patterns

public class BackgroundProcessor
{
    private CancellationTokenSource? _cts;

    public async Task StartAsync()
    {
        _cts = new CancellationTokenSource();
        await ProcessLoopAsync(_cts.Token);
    }

    public void Stop()
    {
        _cts?.Cancel();
    }

    private async Task ProcessLoopAsync(CancellationToken cancellationToken)
    {
        while (!cancellationToken.IsCancellationRequested)
        {
            try
            {
                await ProcessBatchAsync(cancellationToken);
                await Task.Delay(1000, cancellationToken);
            }
            catch (OperationCanceledException)
            {
                // Expected when cancelled
                break;
            }
        }
    }

    // Linked cancellation tokens
    public async Task ProcessWithMultipleTokensAsync(
        CancellationToken userToken,
        CancellationToken systemToken)
    {
        using var linkedCts = CancellationTokenSource
            .CreateLinkedTokenSource(userToken, systemToken);

        await DoWorkAsync(linkedCts.Token);
    }
}

Async Streams (IAsyncEnumerable)

Stream data asynchronously using IAsyncEnumerable<T> (C# 8+).

Basic Async Streams

public async IAsyncEnumerable<LogEntry> ReadLogsAsync(
    string filePath,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    await using var stream = File.OpenRead(filePath);
    using var reader = new StreamReader(stream);

    string? line;
    while ((line = await reader.ReadLineAsync(cancellationToken)) != null)
    {
        if (TryParseLog(line, out var entry))
        {
            yield return entry;
        }
    }
}

// Consuming async streams
public async Task ProcessLogsAsync(string filePath)
{
    await foreach (var log in ReadLogsAsync(filePath))
    {
        Console.WriteLine($"{log.Timestamp}: {log.Message}");
    }
}

Advanced Async Stream Patterns

public class DataStreamProcessor
{
    // Async stream with filtering
    public async IAsyncEnumerable<Event> GetEventsAsync(
        DateTime startDate,
        [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        int page = 0;

        while (true)
        {
            var events = await FetchPageAsync(page++, cancellationToken);

            if (events.Count == 0)
                yield break;

            foreach (var evt in events.Where(e => e.Date >= startDate))
            {
                yield return evt;
            }
        }
    }

    // LINQ-style operations on async streams
    public async IAsyncEnumerable<TResult> SelectAsync<TSource, TResult>(
        IAsyncEnumerable<TSource> source,
        Func<TSource, TResult> selector)
    {
        await foreach (var item in source)
        {
            yield return selector(item);
        }
    }

    // Buffering async streams
    public async IAsyncEnumerable<List<T>> BufferAsync<T>(
        IAsyncEnumerable<T> source,
        int bufferSize)
    {
        var buffer = new List<T>(bufferSize);

        await foreach (var item in source)
        {
            buffer.Add(item);

            if (buffer.Count >= bufferSize)
            {
                yield return buffer;
                buffer = new List<T>(bufferSize);
            }
        }

        if (buffer.Count > 0)
        {
            yield return buffer;
        }
    }
}

Parallel Async Operations

Execute multiple async operations concurrently.

Task.WhenAll and Task.WhenAny

public async Task<Summary> GetDashboardDataAsync()
{
    // Start all operations concurrently
    var userTask = GetUserDataAsync();
    var ordersTask = GetOrdersAsync();
    var analyticsTask = GetAnalyticsAsync();

    // Wait for all to complete
    await Task.WhenAll(userTask, ordersTask, analyticsTask);

    return new Summary
    {
        User = await userTask,
        Orders = await ordersTask,
        Analytics = await analyticsTask
    };
}

// Handle partial failures
public async Task<Results> ProcessWithPartialFailuresAsync()
{
    var tasks = new[]
    {
        ProcessTask1Async(),
        ProcessTask2Async(),
        ProcessTask3Async()
    };

    await Task.WhenAll(tasks.Select(async t =>
    {
        try
        {
            await t;
        }
        catch (Exception ex)
        {
            // Log but don't throw
            Console.WriteLine($"Task failed: {ex.Message}");
        }
    }));

    // Collect successful results
    var results = tasks
        .Where(t => t.IsCompletedSuccessfully)
        .Select(t => t.Result)
        .ToList();

    return new Results { Successful = results };
}

Task.WhenAny for Timeouts and Racing

public async Task<T> WithTimeoutAsync<T>(Task<T> task, TimeSpan timeout)
{
    var delayTask = Task.Delay(timeout);
    var completedTask = await Task.WhenAny(task, delayTask);

    if (completedTask == delayTask)
    {
        throw new TimeoutException("Operation timed out");
    }

    return await task;
}

// Racing multiple sources
public async Task<Data> GetFastestDataAsync()
{
    var primaryTask = GetFromPrimaryAsync();
    var secondaryTask = GetFromSecondaryAsync();
    var cacheTask = GetFromCacheAsync();

    var completedTask = await Task.WhenAny(primaryTask, secondaryTask, cacheTask);
    return await completedTask;
}

// Throttled parallel processing
public async Task<List<Result>> ProcessWithThrottlingAsync(
    IEnumerable<Item> items,
    int maxConcurrency)
{
    var semaphore = new SemaphoreSlim(maxConcurrency);
    var tasks = items.Select(async item =>
    {
        await semaphore.WaitAsync();
        try
        {
            return await ProcessItemAsync(item);
        }
        finally
        {
            semaphore.Release();
        }
    });

    return (await Task.WhenAll(tasks)).ToList();
}

Exception Handling in Async Code

Proper exception handling patterns for async methods.

Basic Exception Handling

public async Task<Result> ProcessWithErrorHandlingAsync()
{
    try
    {
        var data = await FetchDataAsync();
        return await ProcessDataAsync(data);
    }
    catch (HttpRequestException ex)
    {
        _logger.LogError(ex, "Network error occurred");
        throw;
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, "Unexpected error occurred");
        return Result.Failed(ex.Message);
    }
}

// Exception handling with Task.WhenAll
public async Task ProcessMultipleAsync()
{
    var tasks = new[] { Task1Async(), Task2Async(), Task3Async() };

    try
    {
        await Task.WhenAll(tasks);
    }
    catch (Exception ex)
    {
        // Only first exception is thrown
        _logger.LogError(ex, "At least one task failed");

        // To get all exceptions:
        var exceptions = tasks
            .Where(t => t.IsFaulted)
            .Select(t => t.Exception)
            .ToList();

        foreach (var exception in exceptions)
        {
            _logger.LogError(exception, "Task failed");
        }
    }
}

AggregateException Handling

public async Task HandleAllExceptionsAsync()
{
    var tasks = Enumerable.Range(1, 10)
        .Select(i => ProcessItemAsync(i))
        .ToArray();

    try
    {
        await Task.WhenAll(tasks);
    }
    catch
    {
        // Examine all exceptions
        var aggregateException = new AggregateException(
            tasks.Where(t => t.IsFaulted)
                .SelectMany(t => t.Exception?.InnerExceptions ?? Array.Empty<Exception>())
        );

        aggregateException.Handle(ex =>
        {
            if (ex is HttpRequestException)
            {
                _logger.LogWarning(ex, "Network error - retrying");
                return true; // Handled
            }
            return false; // Rethrow
        });
    }
}

Deadlock Prevention

Avoid common deadlock scenarios in async code.

Common Deadlock Patterns

// ❌ DEADLOCK - blocking on async code
public void DeadlockExample()
{
    // This will deadlock in UI or ASP.NET contexts
    var result = GetDataAsync().Result;

    // This will also deadlock
    GetDataAsync().Wait();
}

// ✅ CORRECT - async all the way
public async Task CorrectExample()
{
    var result = await GetDataAsync();
}

// ✅ CORRECT - use ConfigureAwait(false) in library code
public async Task<Data> LibraryMethodAsync()
{
    var data = await FetchAsync().ConfigureAwait(false);
    return ProcessData(data);
}

Avoiding Deadlocks

public class DeadlockFreeService
{
    // ✅ Async all the way
    public async Task<Result> ProcessAsync()
    {
        var data = await GetDataAsync();
        var processed = await ProcessDataAsync(data);
        return processed;
    }

    // ✅ If you must block, use Task.Run
    public Result ProcessSync()
    {
        return Task.Run(async () => await ProcessAsync()).GetAwaiter().GetResult();
    }

    // ✅ Use async disposal
    public async Task UseResourceAsync()
    {
        await using var resource = new AsyncDisposableResource();
        await resource.ProcessAsync();
    }
}

Async in ASP.NET Core

Best practices for async code in ASP.NET Core applications.

Controller Async Patterns

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    private readonly IProductRepository _repository;

    // ✅ Async action methods
    [HttpGet("{id}")]
    public async Task<ActionResult<Product>> GetProduct(
        int id,
        CancellationToken cancellationToken)
    {
        var product = await _repository.GetByIdAsync(id, cancellationToken);

        if (product == null)
            return NotFound();

        return Ok(product);
    }

    [HttpPost]
    public async Task<ActionResult<Product>> CreateProduct(
        [FromBody] CreateProductRequest request,
        CancellationToken cancellationToken)
    {
        var product = await _repository.CreateAsync(request, cancellationToken);
        return CreatedAtAction(nameof(GetProduct), new { id = product.Id }, product);
    }

    // ✅ Streaming responses with IAsyncEnumerable
    [HttpGet("stream")]
    public async IAsyncEnumerable<Product> StreamProducts(
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        await foreach (var product in _repository.GetAllStreamAsync(cancellationToken))
        {
            yield return product;
        }
    }
}

Background Services

public class DataProcessorService : BackgroundService
{
    private readonly IServiceProvider _serviceProvider;
    private readonly ILogger<DataProcessorService> _logger;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Data processor service starting");

        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                await ProcessDataBatchAsync(stoppingToken);
                await Task.Delay(TimeSpan.FromMinutes(5), stoppingToken);
            }
            catch (OperationCanceledException)
            {
                // Expected when stopping
                break;
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Error processing data batch");
                await Task.Delay(TimeSpan.FromSeconds(30), stoppingToken);
            }
        }

        _logger.LogInformation("Data processor service stopped");
    }

    private async Task ProcessDataBatchAsync(CancellationToken cancellationToken)
    {
        using var scope = _serviceProvider.CreateScope();
        var repository = scope.ServiceProvider.GetRequiredService<IDataRepository>();

        await repository.ProcessBatchAsync(cancellationToken);
    }
}

Best Practices

Async All the Way: Never block on async code with .Result or .Wait()
Use CancellationToken: Always accept CancellationToken for long-running operations
ConfigureAwait in Libraries: Use ConfigureAwait(false) in library code
Avoid Async Void: Only use async void for event handlers
Return Task Directly: When possible, return the Task directly without await
Use ValueTask for Hot Paths: Consider ValueTask for frequently-called, often-synchronous methods
Handle All Exceptions: Always handle exceptions in async methods
Don't Mix Blocking and Async: Choose one paradigm per call chain
Dispose Async Resources: Use await using for IAsyncDisposable
Test with Cancellation: Test that cancellation works correctly

Common Pitfalls

Blocking on Async Code: Using .Result or .Wait() causes deadlocks
Forgetting ConfigureAwait: Can cause performance issues in libraries
Async Void Methods: Cannot be awaited and swallow exceptions
Not Handling Cancellation: Ignoring CancellationToken parameter
Over-using Task.Run: Don't wrap already-async code in Task.Run
Capturing Context Unnecessarily: Wastes resources when context not needed
Fire and Forget: Starting async operations without awaiting
Mixing Sync and Async: Creates confusion and potential deadlocks
Not Using ValueTask Correctly: Awaiting ValueTask multiple times
Ignoring Exceptions in Task.WhenAll: Only catching first exception

When to Use

Use this skill when:

Writing asynchronous code in C#
Implementing I/O-bound operations (database, network, file system)
Building responsive UI applications
Creating scalable web services
Working with streams of data
Implementing cancellation support
Optimizing async performance with ValueTask
Handling parallel async operations
Preventing deadlocks in async code
Working with ASP.NET Core async patterns

Resources

Async/Await Best Practices
ConfigureAwait FAQ
Async Streams Tutorial
ValueTask Overview
Task-based Asynchronous Pattern

csharp-async-patterns