c# - Create a completed Task<T>

Question

I'm implementing a method Task<Result> StartSomeTask() and happen to know the result already before the method is called. How do I create a Task<T> that has already completed?

This is what I'm currently doing:

private readonly Result theResult = new Result();

public override Task<Result> StartSomeTask()
{
    var task = new Task<Result>(() => theResult);
    task.RunSynchronously(CurrentThreadTaskScheduler.CurrentThread);
    return task;
}

Is there a better solution?

Answer 1

When targeting .NET 4.5 you can use Task.FromResult:

public static Task<TResult> FromResult<TResult>(TResult result);

To create a failed task, use Task.FromException:

public static Task FromException(Exception exception);
public static Task<TResult> FromException<TResult>(Exception exception);

.NET 4.6 adds Task.CompletedTask if you need a non generic Task.

public static Task CompletedTask { get; }

---

Workarounds for older versions of .NET:

• When targeting .NET 4.0 with Async Targetting Pack (or AsyncCTP) you can use TaskEx.FromResult instead.

• To get non-generic Task prior to .NET 4.6, you can use the fact that Task<T> derives from Task and just call Task.FromResult<object>(null) or Task.FromResult(0).