AsyncAwaitBestPractices

Available on NuGet: https://www.nuget.org/packages/AsyncAwaitBestPractices/

• SafeFireAndForget
  • An extension method to safely fire-and-forget a Task or a ValueTask
  • Ensures the Task will rethrow an Exception if an Exception is caught in IAsyncStateMachine.MoveNext()
• WeakEventManager
  • Avoids memory leaks when events are not unsubscribed
  • Used by AsyncCommand, AsyncCommand<T>, AsyncValueCommand, AsyncValueCommand<T>
• Usage instructions

Setup

• Available on NuGet: https://www.nuget.org/packages/AsyncAwaitBestPractices/
• Add to any project supporting .NET Standard 1.0

Usage

SafeFireAndForget

An extension method to safely fire-and-forget a Task.

SafeFireAndForget allows a Task to safely run on a different thread while the calling thread does not wait for its completion.

public static async void SafeFireAndForget(this System.Threading.Tasks.Task task, System.Action<System.Exception>? onException = null, bool continueOnCapturedContext = false)

public static async void SafeFireAndForget(this System.Threading.Tasks.ValueTask task, System.Action<System.Exception>? onException = null, bool continueOnCapturedContext = false)

Basic Usage - Task

void HandleButtonTapped(object sender, EventArgs e)
{
    // Allows the async Task method to safely run on a different thread while the calling thread continues, not awaiting its completion
    // onException: If an Exception is thrown, print it to the Console
    ExampleAsyncMethod().SafeFireAndForget(onException: ex => Console.WriteLine(ex));

    // HandleButtonTapped continues execution here while `ExampleAsyncMethod()` is running on a different thread
    // ...
}

async Task ExampleAsyncMethod()
{
    await Task.Delay(1000);
}

Basic Usage - ValueTask

If you're new to ValueTask, check out this great write-up, Understanding the Whys, Whats, and Whens of ValueTask.

void HandleButtonTapped(object sender, EventArgs e)
{
    // Allows the async ValueTask method to safely run on a different thread while the calling thread continues, not awaiting its completion
    // onException: If an Exception is thrown, print it to the Console
    ExampleValueTaskMethod().SafeFireAndForget(onException: ex => Console.WriteLine(ex));

    // HandleButtonTapped continues execution here while `ExampleAsyncMethod()` is running on a different thread
    // ...
}

async ValueTask ExampleValueTaskMethod()
{
    var random = new Random();
    if (random.Next(10) > 9)
        await Task.Delay(1000);
}

Advanced Usage

void InitializeSafeFireAndForget()
{
    // Initialize SafeFireAndForget
    // Only use `shouldAlwaysRethrowException: true` when you want `.SafeFireAndForget()` to always rethrow every exception. This is not recommended, because there is no way to catch an Exception rethrown by `SafeFireAndForget()`; `shouldAlwaysRethrowException: true` should **not** be used in Production/Release builds.
    SafeFireAndForgetExtensions.Initialize(shouldAlwaysRethrowException: false);

    // SafeFireAndForget will print every exception to the Console
    SafeFireAndForgetExtensions.SetDefaultExceptionHandling(ex => Console.WriteLine(ex));
}

void UninitializeSafeFireAndForget()
{
    // Remove default exception handling
    SafeFireAndForgetExtensions.RemoveDefaultExceptionHandling()
}

void HandleButtonTapped(object sender, EventArgs e)
{
    // Allows the async Task method to safely run on a different thread while not awaiting its completion
    // onException: If a WebException is thrown, print its StatusCode to the Console. **Note**: If a non-WebException is thrown, it will not be handled by `onException`
    // Because we set `SetDefaultExceptionHandling` in `void InitializeSafeFireAndForget()`, the entire exception will also be printed to the Console
    ExampleAsyncMethod().SafeFireAndForget<WebException>(onException: ex =>
    {
        if(ex.Response is HttpWebResponse webResponse)
            Console.WriteLine($"Task Exception\n Status Code: {webResponse.StatusCode}");
    });
    
    ExampleValueTaskMethod().SafeFireAndForget<WebException>(onException: ex =>
    {
        if(ex.Response is HttpWebResponse webResponse)
            Console.WriteLine($"ValueTask Error\n Status Code: {webResponse.StatusCode}");
    });

    // HandleButtonTapped continues execution here while `ExampleAsyncMethod()` and `ExampleValueTaskMethod()` run in the background
}

async Task ExampleAsyncMethod()
{
    await Task.Delay(1000);
    throw new WebException();
}

async ValueTask ExampleValueTaskMethod()
{
    var random = new Random();
    if (random.Next(10) > 9)
        await Task.Delay(1000);
        
    throw new WebException();
}

WeakEventManager

An event implementation that enables the garbage collector to collect an object without needing to unsubscribe event handlers.

Inspired by Xamarin.Forms.WeakEventManager.

Using EventHandler

readonly WeakEventManager _canExecuteChangedEventManager = new WeakEventManager();

public event EventHandler CanExecuteChanged
{
    add => _canExecuteChangedEventManager.AddEventHandler(value);
    remove => _canExecuteChangedEventManager.RemoveEventHandler(value);
}

void OnCanExecuteChanged() => _canExecuteChangedEventManager.RaiseEvent(this, EventArgs.Empty, nameof(CanExecuteChanged));

Using Delegate

readonly WeakEventManager _propertyChangedEventManager = new WeakEventManager();

public event PropertyChangedEventHandler PropertyChanged
{
    add => _propertyChangedEventManager.AddEventHandler(value);
    remove => _propertyChangedEventManager.RemoveEventHandler(value);
}

void OnPropertyChanged([CallerMemberName]string propertyName = "") => _propertyChangedEventManager.RaiseEvent(this, new PropertyChangedEventArgs(propertyName), nameof(PropertyChanged));

Using Action

readonly WeakEventManager _weakActionEventManager = new WeakEventManager();

public event Action ActionEvent
{
    add => _weakActionEventManager.AddEventHandler(value);
    remove => _weakActionEventManager.RemoveEventHandler(value);
}

void OnActionEvent(string message) => _weakActionEventManager.RaiseEvent(message, nameof(ActionEvent));

WeakEventManager<T>

An event implementation that enables the garbage collector to collect an object without needing to unsubscribe event handlers.

Inspired by Xamarin.Forms.WeakEventManager.

Using EventHandler<T>

readonly WeakEventManager<string> _errorOcurredEventManager = new WeakEventManager<string>();

public event EventHandler<string> ErrorOcurred
{
    add => _errorOcurredEventManager.AddEventHandler(value);
    remove => _errorOcurredEventManager.RemoveEventHandler(value);
}

void OnErrorOcurred(string message) => _errorOcurredEventManager.RaiseEvent(this, message, nameof(ErrorOcurred));

Using Action<T>

readonly WeakEventManager<string> _weakActionEventManager = new WeakEventManager<string>();

public event Action<string> ActionEvent
{
    add => _weakActionEventManager.AddEventHandler(value);
    remove => _weakActionEventManager.RemoveEventHandler(value);
}

void OnActionEvent(string message) => _weakActionEventManager.RaiseEvent(message, nameof(ActionEvent));