Create a completed Task<T>

Question

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; }

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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).

private readonly Result theResult = new Result();

public override Task<Result> StartSomeTask()
{
    var taskSource = new TaskCompletionSource<Result>();
    taskSource.SetResult(theResult);
    return taskSource.Task;
}

For tasks with no return value, .NET 4.6 has added Task.CompletedTask.

It returns a task which is already in TaskStatus.RanToCompletion. It probably returns the same instance every time, but the documentation warns you not to count on that fact.

If you're using Rx, an alternative is Observable.Return(result).ToTask().

Calling Task.WhenAll without any parameters will return a completed task.

Task task = Task.WhenAll();