ImplementingAsyncHandler.md

How to Implement an Asynchronous Request Handler

To implement an asynchronous handler, derive from RequestHandlerAsync<T> where T should be the Command or Event derived type that you wish to handle. Then override the base class RequestHandlerAsync<T>.HandleAsync() method to implement your handling for the Command or Event.

For example, assume that you want to handle the Command GreetingCommand

public class GreetingCommand : Command
{
    public GreetingCommand(string name)
        : base(Guid.NewGuid())
    {
        Name = name;
    }

    public Guid Id { get; set; }
    public string Name { get; private set; }
}

Then derive your handler from RequestHandlerAsync<GreetingCommand> and accept a parameter of that type on the overriden HandleAsync() method, along with a nullable cancellation token - which you should default to null.

To ensure that the pipeline runs, you should return the result of the next handler in the chain, by awaiting the base class HandleAsync().

(Because the next element in the pipeline should also be async, you should always await the result of this call.)

public class GreetingCommandRequestHandlerAsync : RequestHandlerAsync<GreetingCommand>
{
    public override async Task HandleAsync(GreetingCommand command, CancellationToken? ct = null)
    {
        var api = new IpFyApi(new Uri("https://api.ipify.org"));

        var result = await api.GetAsync(ct);

        Console.WriteLine("Hello {0}", command.Name);
        Console.WriteLine(result.Success ? "Your public IP addres is {0}" : "Call to IpFy API failed : {0}",
        result.Message);
        return await base.HandleAsync(command, ct).ConfigureAwait(base.ContinueOnCapturedContext);
    }
}

Note how we use ConfigureAwait() when calling the next handler in the chain, and set the value to the RequestHandlerAsync<GreetingCommand>.ContinueOnCapturedContext property. This ensures that we utilize any override of the default (which is to use the Task Scheduler) made when the call to SendAsync, PublishAsync, or PostAsync was made.

It is worth noting that although the override forces you to return a Task<T> it does not force you to add the async keyword to the method to compile. This risks introducing a subtle bug. You can await a method that returns a Task<T> but creation of the state machine in the caller depends on the presence of the async keyword. If your handler does not await anything, you will not be forced to add the async keyword. Your handler will run sychronously in this context, which may not be what you expect.

Remembering to always await the base class HandleAsync() mitigates against this as even if your handler does not do asynchronous work, you will be forced to add async to the signature.