Asynchronous Programming

Asynchronous programming has been around for several years on the .NET platform but has historically been very difficult to do well. Since the introduction of async/await in C# 5 asynchronous programming has become mainstream. Modern frameworks (like ASP.NET Core) are fully asynchronous and it's very hard to avoid the async keyword when writing web services. As a result, there's been lots of confusion on the best practices for async and how to use it properly. This section will try to lay out some guidance with examples of bad and good patterns of how to write asynchronous code.

Asynchrony is viral

Once you go async, all of your callers SHOULD be async, since efforts to be async amount to nothing unless the entire callstack is async. In many cases, being partially async can be worse than being entirely synchronous. Therefore it is best to go all in, and make everything async at once.

❌ BAD This example uses the Task.Result and as a result blocks the current thread to wait for the result. This is an example of sync over async.

public int DoSomethingAsync()
{
    var result = CallDependencyAsync().Result;
    return result + 1;
}

✅ GOOD This example uses the await keyword to get the result from CallDependencyAsync.

public async Task<int> DoSomethingAsync()
{
    var result = await CallDependencyAsync();
    return result + 1;
}

Async void

Use of async void in ASP.NET Core applications is ALWAYS bad. Avoid it, never do it. Typically, it's used when developers are trying to implement fire and forget patterns triggered by a controller action. Async void methods will crash the process if an exception is thrown. We'll look at more of the patterns that cause developers to do this in ASP.NET Core applications but here's a simple example:

❌ BAD Async void methods can't be tracked and therefore unhandled exceptions can result in application crashes.

public class MyController : Controller
{
    [HttpPost("/start")]
    public IActionResult Post()
    {
        BackgroundOperationAsync();
        return Accepted();
    }
    
    public async void BackgroundOperationAsync()
    {
        var result = await CallDependencyAsync();
        DoSomething(result);
    }
}

✅ GOOD Task-returning methods are better since unhandled exceptions trigger the TaskScheduler.UnobservedTaskException.

public class MyController : Controller
{
    [HttpPost("/start")]
    public IActionResult Post()
    {
        Task.Run(BackgroundOperationAsync);
        return Accepted();
    }
    
    public async Task BackgroundOperationAsync()
    {
        var result = await CallDependencyAsync();
        DoSomething(result);
    }
}

Prefer `Task.FromResult` over `Task.Run` for pre-computed or trivially computed data

For pre-computed results, there's no need to call Task.Run, that will end up queuing a work item to the thread pool that will immediately complete with the pre-computed value. Instead, use Task.FromResult, to create a task wrapping already computed data.

❌ BAD This example wastes a thread-pool thread to return a trivially computed value.

public class MyLibrary
{
   public Task<int> AddAsync(int a, int b)
   {
       return Task.Run(() => a + b);
   }
}

✅ GOOD This example uses Task.FromResult to return the trivially computed value. It does not use any extra threads as a result.

public class MyLibrary
{
   public Task<int> AddAsync(int a, int b)
   {
       return Task.FromResult(a + b);
   }
}

💡NOTE: Using Task.FromResult will result in a Task allocation. Using ValueTask<T> can completely remove that allocation.

✅ GOOD This example uses a ValueTask<int> to return the trivially computed value. It does not use any extra threads as a result. It also does not allocate an object on the managed heap.

public class MyLibrary
{
   public ValueTask<int> AddAsync(int a, int b)
   {
       return new ValueTask<int>(a + b);
   }
}

Avoid using Task.Run for long running work that blocks the thread

Long running work in this context refers to a thread that's running for the lifetime of the application doing background work (like processing queue items, or sleeping and waking up to process some data). Task.Run will queue a work item to the thread pool. The assumption is that that work will finish quickly (or quickly enough to allow reusing that thread within some reasonable timeframe). Stealing a thread-pool thread for long-running work is bad since it takes that thread away from other work that could be done (timer callbacks, task continuations etc). Instead, spawn a new thread manually to do long running blocking work.

💡 NOTE: The thread pool grows if you block threads but it's bad practice to do so.

💡 NOTE:Task.Factory.StartNew has an option TaskCreationOptions.LongRunning that under the covers creates a new thread and returns a Task that represents the execution. Using this properly requires several non-obvious parameters to be passed in to get the right behavior on all platforms.

💡 NOTE: Don't use TaskCreationOptions.LongRunning with async code as this will create a new thread which will be destroyed after first await.

❌ BAD This example steals a thread-pool thread forever, to execute queued work on a BlockingCollection<T>.

public class QueueProcessor
{
    private readonly BlockingCollection<Message> _messageQueue = new BlockingCollection<Message>();
    
    public void StartProcessing()
    {
        Task.Run(ProcessQueue);
    }
    
    public void Enqueue(Message message)
    {
        _messageQueue.Add(message);
    }
    
    private void ProcessQueue()
    {
        foreach (var item in _messageQueue.GetConsumingEnumerable())
        {
             ProcessItem(item);
        }
    }
    
    private void ProcessItem(Message message) { }
}

✅ GOOD This example uses a dedicated thread to process the message queue instead of a thread-pool thread.

public class QueueProcessor
{
    private readonly BlockingCollection<Message> _messageQueue = new BlockingCollection<Message>();
    
    public void StartProcessing()
    {
        var thread = new Thread(ProcessQueue) 
        {
            // This is important as it allows the process to exit while this thread is running
            IsBackground = true
        };
        thread.Start();
    }
    
    public void Enqueue(Message message)
    {
        _messageQueue.Add(message);
    }
    
    private void ProcessQueue()
    {
        foreach (var item in _messageQueue.GetConsumingEnumerable())
        {
             ProcessItem(item);
        }
    }
    
    private void ProcessItem(Message message) { }
}

Avoid using `Task.Result` and `Task.Wait`

There are very few ways to use Task.Result and Task.Wait correctly so the general advice is to completely avoid using them in your code.

⚠️ Sync over `async`

Using Task.Result or Task.Wait to block wait on an asynchronous operation to complete is MUCH worse than calling a truly synchronous API to block. This phenomenon is dubbed "Sync over async". Here is what happens at a very high level:

An asynchronous operation is kicked off.
The calling thread is blocked waiting for that operation to complete.
When the asynchronous operation completes, it unblocks the code waiting on that operation. This takes place on another thread.

The result is that we need to use 2 threads instead of 1 to complete synchronous operations. This usually leads to thread-pool starvation and results in service outages.

⚠️ Deadlocks

The SynchronizationContext is an abstraction that gives application models a chance to control where asynchronous continuations run. ASP.NET (non-core), WPF and Windows Forms each have an implementation that will result in a deadlock if Task.Wait or Task.Result is used on the main thread. This behavior has led to a bunch of "clever" code snippets that show the "right" way to block waiting for a Task. The truth is, there's no good way to block waiting for a Task to complete.

💡NOTE: ASP.NET Core does not have a SynchronizationContext and is not prone to the deadlock problem.

❌ BAD The below are all examples that are, in one way or another, trying to avoid the deadlock situation but still succumb to "sync over async" problems.

public string DoOperationBlocking()
{
    // Bad - Blocking the thread that enters.
    // DoAsyncOperation will be scheduled on the default task scheduler, and remove the risk of deadlocking.
    // In the case of an exception, this method will throw an AggregateException wrapping the original exception.
    return Task.Run(() => DoAsyncOperation()).Result;
}

public string DoOperationBlocking2()
{
    // Bad - Blocking the thread that enters.
    // DoAsyncOperation will be scheduled on the default task scheduler, and remove the risk of deadlocking.
    return Task.Run(() => DoAsyncOperation()).GetAwaiter().GetResult();
}

public string DoOperationBlocking3()
{
    // Bad - Blocking the thread that enters, and blocking the theadpool thread inside.
    // In the case of an exception, this method will throw an AggregateException containing another AggregateException, containing the original exception.
    return Task.Run(() => DoAsyncOperation().Result).Result;
}

public string DoOperationBlocking4()
{
    // Bad - Blocking the thread that enters, and blocking the theadpool thread inside.
    return Task.Run(() => DoAsyncOperation().GetAwaiter().GetResult()).GetAwaiter().GetResult();
}

public string DoOperationBlocking5()
{
    // Bad - Blocking the thread that enters.
    // Bad - No effort has been made to prevent a present SynchonizationContext from becoming deadlocked.
    // In the case of an exception, this method will throw an AggregateException wrapping the original exception.
    return DoAsyncOperation().Result;
}

public string DoOperationBlocking6()
{
    // Bad - Blocking the thread that enters.
    // Bad - No effort has been made to prevent a present SynchonizationContext from becoming deadlocked.
    return DoAsyncOperation().GetAwaiter().GetResult();
}

public string DoOperationBlocking7()
{
    // Bad - Blocking the thread that enters.
    // Bad - No effort has been made to prevent a present SynchonizationContext from becoming deadlocked.
    var task = DoAsyncOperation();
    task.Wait();
    return task.GetAwaiter().GetResult();
}

Prefer `await` over `ContinueWith`

Task existed before the async/await keywords were introduced and as such provided ways to execute continuations without relying on the language. Although these methods are still valid to use, we generally recommend that you prefer async/await to using ContinueWith. ContinueWith also does not capture the SynchronizationContext and as a result is actually semantically different to async/await.

❌ BAD The example uses ContinueWith instead of async

public Task<int> DoSomethingAsync()
{
    return CallDependencyAsync().ContinueWith(task =>
    {
        return task.Result + 1;
    });
}

✅ GOOD This example uses the await keyword to get the result from CallDependencyAsync.

public async Task<int> DoSomethingAsync()
{
    var result = await CallDependencyAsync();
    return result + 1;
}

Always create `TaskCompletionSource<T>` with `TaskCreationOptions.RunContinuationsAsynchronously`

TaskCompletionSource<T> is an important building block for libraries trying to adapt things that are not inherently awaitable to be awaitable via a Task. It is also commonly used to build higher-level operations (such as batching and other combinators) on top of existing asynchronous APIs. By default, Task continuations will run inline on the same thread that calls Try/Set(Result/Exception/Canceled). As a library author, this means having to understand that calling code can resume directly on your thread. This is extremely dangerous and can result in deadlocks, thread-pool starvation, corruption of state (if code runs unexpectedly) and more.

Always use TaskCreationOptions.RunContinuationsAsynchronously when creating the TaskCompletionSource<T>. This will dispatch the continuation onto the thread pool instead of executing it inline.

❌ BAD This example does not use TaskCreationOptions.RunContinuationsAsynchronously when creating the TaskCompletionSource<T>.

public Task<int> DoSomethingAsync()
{
    var tcs = new TaskCompletionSource<int>();
    
    var operation = new LegacyAsyncOperation();
    operation.Completed += result =>
    {
        // Code awaiting on this task will resume on this thread!
        tcs.SetResult(result);
    };
    
    return tcs.Task;
}

✅ GOOD This example uses TaskCreationOptions.RunContinuationsAsynchronously when creating the TaskCompletionSource<T>.

public Task<int> DoSomethingAsync()
{
    var tcs = new TaskCompletionSource<int>(TaskCreationOptions.RunContinuationsAsynchronously);
    
    var operation = new LegacyAsyncOperation();
    operation.Completed += result =>
    {
        // Code awaiting on this task will resume on a different thread-pool thread
        tcs.SetResult(result);
    };
    
    return tcs.Task;
}

💡NOTE: There are 2 enums that look alike. TaskCreationOptions.RunContinuationsAsynchronously and TaskContinuationOptions.RunContinuationsAsynchronously. Be careful not to confuse their usage.

Always dispose `CancellationTokenSource`(s) used for timeouts

CancellationTokenSource objects that are used for timeouts (are created with timers or uses the CancelAfter method), can put pressure on the timer queue if not disposed.

❌ BAD This example does not dispose the CancellationTokenSource and as a result the timer stays in the queue for 10 seconds after each request is made.

public async Task<Stream> HttpClientAsyncWithCancellationBad()
{
    var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));

    using (var client = _httpClientFactory.CreateClient())
    {
        var response = await client.GetAsync("http://backend/api/1", cts.Token);
        return await response.Content.ReadAsStreamAsync();
    }
}

✅ GOOD This example disposes the CancellationTokenSource and properly removes the timer from the queue.

public async Task<Stream> HttpClientAsyncWithCancellationGood()
{
    using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
    {
        using (var client = _httpClientFactory.CreateClient())
        {
            var response = await client.GetAsync("http://backend/api/1", cts.Token);
            return await response.Content.ReadAsStreamAsync();
        }
    }
}

Always flow `CancellationToken`(s) to APIs that take a `CancellationToken`

Cancellation is cooperative in .NET. Everything in the call-chain has to be explicitly passed the CancellationToken in order for it to work well. This means you need to explicitly pass the token into other APIs that take a token if you want cancellation to be most effective.

❌ BAD This example neglects to pass the CancellationToken to Stream.ReadAsync making the operation effectively not cancellable.

public async Task<string> DoAsyncThing(CancellationToken cancellationToken = default)
{
   byte[] buffer = new byte[1024];
   // We forgot to pass flow cancellationToken to ReadAsync
   int read = await _stream.ReadAsync(buffer, 0, buffer.Length);
   return Encoding.UTF8.GetString(buffer, 0, read);
}

✅ GOOD This example passes the CancellationToken into Stream.ReadAsync.

public async Task<string> DoAsyncThing(CancellationToken cancellationToken = default)
{
   byte[] buffer = new byte[1024];
   // This properly flows cancellationToken to ReadAsync
   int read = await _stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken);
   return Encoding.UTF8.GetString(buffer, 0, read);
}

Cancelling uncancellable operations

One of the coding patterns that appears when doing asynchronous programming is cancelling an uncancellable operation. This usually means creating another task that completes when a timeout or CancellationToken fires, and then using Task.WhenAny to detect a complete or cancelled operation.

Using CancellationTokens

❌ BAD This example uses Task.Delay(-1, token) to create a Task that completes when the CancellationToken fires, but if it doesn't fire, there's no way to dispose the CancellationTokenRegistration. This can lead to a memory leak.

public static async Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken)
{
    // There's no way to dispose the registration
    var delayTask = Task.Delay(-1, cancellationToken);

    var resultTask = await Task.WhenAny(task, delayTask);
    if (resultTask == delayTask)
    {
        // Operation cancelled
        throw new OperationCanceledException();
    }

    return await task;
}

✅ GOOD This example disposes the CancellationTokenRegistration when one of the Task(s) complete.

public static async Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken)
{
    var tcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);

    // This disposes the registration as soon as one of the tasks trigger
    using (cancellationToken.Register(state =>
    {
        ((TaskCompletionSource<object>)state).TrySetResult(null);
    },
    tcs))
    {
        var resultTask = await Task.WhenAny(task, tcs.Task);
        if (resultTask == tcs.Task)
        {
            // Operation cancelled
            throw new OperationCanceledException(cancellationToken);
        }

        return await task;
    }
}

Using a timeout

❌ BAD This example does not cancel the timer even if the operation successfuly completes. This means you could end up with lots of timers, which can flood the timer queue.

public static async Task<T> TimeoutAfter<T>(this Task<T> task, TimeSpan timeout)
{
    var delayTask = Task.Delay(timeout);

    var resultTask = await Task.WhenAny(task, delayTask);
    if (resultTask == delayTask)
    {
        // Operation cancelled
        throw new OperationCanceledException();
    }

    return await task;
}

✅ GOOD This example cancels the timer if the operation succesfully completes.

public static async Task<T> TimeoutAfter<T>(this Task<T> task, TimeSpan timeout)
{
    using (var cts = new CancellationTokenSource())
    {
        var delayTask = Task.Delay(timeout, cts.Token);

        var resultTask = await Task.WhenAny(task, delayTask);
        if (resultTask == delayTask)
        {
            // Operation cancelled
            throw new OperationCanceledException();
        }
        else
        {
            // Cancel the timer task so that it does not fire
            cts.Cancel();
        }

        return await task;
    }
}

Always call `FlushAsync` on `StreamWriter`(s) or `Stream`(s) before calling `Dispose`

When writing to a Stream or StreamWriter, even if the asynchronous overloads are used for writing, the underlying data might be buffered. When data is buffered, disposing the Stream or StreamWriter via the Dispose method will synchronously write/flush, which results in blocking the thread and could lead to thread-pool starvation. Either use the asynchronous DisposeAsync method (for example via await using) or call FlushAsync before calling Dispose.

💡NOTE: This is only problematic if the underlying subsystem does IO.

❌ BAD This example ends up blocking the request by writing synchronously to the HTTP-response body.

app.Run(async context =>
{
    // The implicit Dispose call will synchronously write to the response body
    using (var streamWriter = new StreamWriter(context.Response.Body))
    {
        await streamWriter.WriteAsync("Hello World");
    }
});

✅ GOOD This example asynchronously flushes any buffered data while disposing the StreamWriter.

app.Run(async context =>
{
    // The implicit AsyncDispose call will flush asynchronously
    await using (var streamWriter = new StreamWriter(context.Response.Body))
    {
        await streamWriter.WriteAsync("Hello World");
    }
});

✅ GOOD This example asynchronously flushes any buffered data before disposing the StreamWriter.

app.Run(async context =>
{
    using (var streamWriter = new StreamWriter(context.Response.Body))
    {
        await streamWriter.WriteAsync("Hello World");

        // Force an asynchronous flush
        await streamWriter.FlushAsync();
    }
});

Prefer `async`/`await` over directly returning `Task`

There are benefits to using the async/await keyword instead of directly returning the Task:

Asynchronous and synchronous exceptions are normalized to always be asynchronous.
The code is easier to modify (consider adding a using, for example).
Diagnostics of asynchronous methods are easier (debugging hangs etc).
Exceptions thrown will be automatically wrapped in the returned Task instead of surprising the caller with an actual exception.

❌ BAD This example directly returns the Task to the caller.

public Task<int> DoSomethingAsync()
{
    return CallDependencyAsync();
}

✅ GOOD This examples uses async/await instead of directly returning the Task.

public async Task<int> DoSomethingAsync()
{
    return await CallDependencyAsync();
}

💡NOTE: There are performance considerations when using an async state machine over directly returning the Task. It's always faster to directly return the Task since it does less work but you end up changing the behavior and potentially losing some of the benefits of the async state machine.

ConfigureAwait

TBD

Scenarios

The above tries to distill general guidance, but doesn't do justice to the kinds of real-world situations that cause code like this to be written in the first place (bad code). This section tries to take concrete examples from real applications and turn them into something simple to help you relate these problems to existing codebases.

`Timer` callbacks

❌ BAD The Timer callback is void-returning and we have asynchronous work to execute. This example uses async void to accomplish it and as a result can crash the process if an exception occurs.

public class Pinger
{
    private readonly Timer _timer;
    private readonly HttpClient _client;
    
    public Pinger(HttpClient client)
    {
        _client = client;
        _timer = new Timer(Heartbeat, null, 1000, 1000);
    }

    public async void Heartbeat(object state)
    {
        await _client.GetAsync("http://mybackend/api/ping");
    }
}

❌ BAD This attempts to block in the Timer callback. This may result in thread-pool starvation and is an example of sync over async

public class Pinger
{
    private readonly Timer _timer;
    private readonly HttpClient _client;
    
    public Pinger(HttpClient client)
    {
        _client = client;
        _timer = new Timer(Heartbeat, null, 1000, 1000);
    }

    public void Heartbeat(object state)
    {
        _client.GetAsync("http://mybackend/api/ping").GetAwaiter().GetResult();
    }
}

✅ GOOD This example uses an async Task-based method and discards the Task in the Timer callback. If this method fails, it will not crash the process. Instead, it will fire the TaskScheduler.UnobservedTaskException event.

public class Pinger
{
    private readonly Timer _timer;
    private readonly HttpClient _client;
    
    public Pinger(HttpClient client)
    {
        _client = client;
        _timer = new Timer(Heartbeat, null, 1000, 1000);
    }

    public void Heartbeat(object state)
    {
        // Discard the result
        _ = DoAsyncPing();
    }

    private async Task DoAsyncPing()
    {
        await _client.GetAsync("http://mybackend/api/ping");
    }
}

Implicit `async void` delegates

Imagine a BackgroundQueue with a FireAndForget that takes a callback. This method will execute the callback at some time in the future.

❌ BAD This will force callers to either block in the callback or use an async void delegate.

public class BackgroundQueue
{
    public static void FireAndForget(Action action) { }
}

❌ BAD This calling code is creating an async void method implicitly. The compiler fully supports this today.

public class Program
{
    public void Main(string[] args)
    {
        var httpClient = new HttpClient();
        BackgroundQueue.FireAndForget(async () =>
        {
            await httpClient.GetAsync("http://pinger/api/1");
        });
        
        Console.ReadLine();
    }
}

✅ GOOD This BackgroundQueue implementation offers both sync and async callback overloads.

public class BackgroundQueue
{
    public static void FireAndForget(Action action) { }
    public static void FireAndForget(Func<Task> action) { }
}

`ConcurrentDictionary.GetOrAdd`

It's pretty common to cache the result of an asynchronous operation and ConcurrentDictionary is a good data structure for doing that. GetOrAdd is a convenience API for trying to get an item if it's already there or adding it if it isn't. The callback is synchronous so it's tempting to write code that uses Task.Result to produce the value of an asynchronous process but that can lead to thread-pool starvation.

❌ BAD This may result in thread-pool starvation since we're blocking the request thread if the person data is not cached.

public class PersonController : Controller
{
   private AppDbContext _db;
   
   // This cache needs expiration
   private static ConcurrentDictionary<int, Person> _cache = new ConcurrentDictionary<int, Person>();
   
   public PersonController(AppDbContext db)
   {
      _db = db;
   }
   
   public IActionResult Get(int id)
   {
       var person = _cache.GetOrAdd(id, (key) => _db.People.FindAsync(key).Result);
       return Ok(person);
   }
}

✅ GOOD This implementation won't result in thread-pool starvation since we're storing a task instead of the result itself.

⚠️ ConcurrentDictionary.GetOrAdd will potentially run the cache callback multiple times in parallel. This can result in kicking off expensive computations multiple times.

public class PersonController : Controller
{
   private AppDbContext _db;
   
   // This cache needs expiration
   private static ConcurrentDictionary<int, Task<Person>> _cache = new ConcurrentDictionary<int, Task<Person>>();
   
   public PersonController(AppDbContext db)
   {
      _db = db;
   }
   
   public async Task<IActionResult> Get(int id)
   {
       var person = await _cache.GetOrAdd(id, (key) => _db.People.FindAsync(key));
       return Ok(person);
   }
}

✅ GOOD This implementation fixes the multiple-executing callback issue by using the async lazy pattern.

public class PersonController : Controller
{
   private AppDbContext _db;
   
   // This cache needs expiration
   private static ConcurrentDictionary<int, AsyncLazy<Person>> _cache = new ConcurrentDictionary<int, AsyncLazy<Person>>();
   
   public PersonController(AppDbContext db)
   {
      _db = db;
   }
   
   public async Task<IActionResult> Get(int id)
   {
       var person = await _cache.GetOrAdd(id, (key) => new AsyncLazy<Person>(() => _db.People.FindAsync(key))).Value;
       return Ok(person);
   }
   
   private class AsyncLazy<T> : Lazy<Task<T>>
   {
      public AsyncLazy(Func<Task<T>> valueFactory) : base(valueFactory)
      {
      }
   }
}

Constructors

Constructors are synchronous. If you need to initialize some logic that may be asynchronous, there are a couple of patterns for dealing with this.

Here's an example of using a client API that needs to connect asynchronously before use.

public interface IRemoteConnectionFactory
{
   Task<IRemoteConnection> ConnectAsync();
}

public interface IRemoteConnection
{
    Task PublishAsync(string channel, string message);
    Task DisposeAsync();
}

❌ BAD This example uses Task.Result to get the connection in the constructor. This could lead to thread-pool starvation and deadlocks.

public class Service : IService
{
    private readonly IRemoteConnection _connection;
    
    public Service(IRemoteConnectionFactory connectionFactory)
    {
        _connection = connectionFactory.ConnectAsync().Result;
    }
}

✅ GOOD This implementation uses a static factory pattern in order to allow asynchronous construction:

public class Service : IService
{
    private readonly IRemoteConnection _connection;

    private Service(IRemoteConnection connection)
    {
        _connection = connection;
    }

    public static async Task<Service> CreateAsync(IRemoteConnectionFactory connectionFactory)
    {
        return new Service(await connectionFactory.ConnectAsync());
    }
}

WindowsIdentity.RunImpersonated

This API runs the specified action as the impersonated Windows identity. Unfortunately there's no asynchronous version of the callback.

❌ BAD This example tries to execute the query asynchronously, and then wait for it outside of the call to RunImpersonated. This will throw because the query might be executing outside of the impersonation context.

public async Task<IEnumerable<Product>> GetDataImpersonatedAsync(SafeAccessTokenHandle safeAccessTokenHandle)
{
    Task<IEnumerable<Product>> products = null;
    WindowsIdentity.RunImpersonated(
        safeAccessTokenHandle,
        context =>
        {
            products = _db.QueryAsync("SELECT Name from Products");
        }};
    return await products;
}

❌ BAD This example uses Task.Result to get the connection in the constructor. This could lead to thread-pool starvation and deadlocks.

public IEnumerable<Product> GetDataImpersonatedAsync(SafeAccessTokenHandle safeAccessTokenHandle)
{
    return WindowsIdentity.RunImpersonated(
        safeAccessTokenHandle,
        context => _db.QueryAsync("SELECT Name from Products").Result);
}

💡NOTE There's no good alternative here. This API shouldn't be used with an asynchronous callback. See https://github.com/dotnet/corefx/issues/24977

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AsyncGuidance.md

AsyncGuidance.md

Table of contents

Asynchronous Programming

Asynchrony is viral

Async void

Prefer `Task.FromResult` over `Task.Run` for pre-computed or trivially computed data

Avoid using Task.Run for long running work that blocks the thread

Avoid using `Task.Result` and `Task.Wait`

⚠️ Sync over `async`

⚠️ Deadlocks

Prefer `await` over `ContinueWith`

Always create `TaskCompletionSource<T>` with `TaskCreationOptions.RunContinuationsAsynchronously`

Always dispose `CancellationTokenSource`(s) used for timeouts

Always flow `CancellationToken`(s) to APIs that take a `CancellationToken`

Cancelling uncancellable operations

Using CancellationTokens

Using a timeout

Always call `FlushAsync` on `StreamWriter`(s) or `Stream`(s) before calling `Dispose`

Prefer `async`/`await` over directly returning `Task`

ConfigureAwait

Scenarios

`Timer` callbacks

Implicit `async void` delegates

`ConcurrentDictionary.GetOrAdd`

Constructors

WindowsIdentity.RunImpersonated

Files

AsyncGuidance.md

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AsyncGuidance.md

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Asynchronous Programming

Asynchrony is viral

Async void

Prefer Task.FromResult over Task.Run for pre-computed or trivially computed data

Avoid using Task.Run for long running work that blocks the thread

Avoid using Task.Result and Task.Wait

⚠️ Sync over async

⚠️ Deadlocks

Prefer await over ContinueWith

Always create TaskCompletionSource<T> with TaskCreationOptions.RunContinuationsAsynchronously

Always dispose CancellationTokenSource(s) used for timeouts

Always flow CancellationToken(s) to APIs that take a CancellationToken

Cancelling uncancellable operations

Using CancellationTokens

Using a timeout

Always call FlushAsync on StreamWriter(s) or Stream(s) before calling Dispose

Prefer async/await over directly returning Task

ConfigureAwait

Scenarios

Timer callbacks

Implicit async void delegates

ConcurrentDictionary.GetOrAdd

Constructors

WindowsIdentity.RunImpersonated

Prefer `Task.FromResult` over `Task.Run` for pre-computed or trivially computed data

Avoid using `Task.Result` and `Task.Wait`

⚠️ Sync over `async`

Prefer `await` over `ContinueWith`

Always create `TaskCompletionSource<T>` with `TaskCreationOptions.RunContinuationsAsynchronously`

Always dispose `CancellationTokenSource`(s) used for timeouts

Always flow `CancellationToken`(s) to APIs that take a `CancellationToken`

Always call `FlushAsync` on `StreamWriter`(s) or `Stream`(s) before calling `Dispose`

Prefer `async`/`await` over directly returning `Task`

`Timer` callbacks

Implicit `async void` delegates

`ConcurrentDictionary.GetOrAdd`