Create a completed Task

My preferred method for doing this is to call Task.WhenAll() with no arguments. The MSDN documentation states that "If the supplied array/enumerable contains no tasks, the returned task will immediately transition to a RanToCompletion state before it's returned to the caller.". That sounds like what you want.

Update: I found the source over at Microsoft's Reference Source; there you can see that Task.WhenAll contains the following:

return (tasks.Length == 0) ? // take shortcut if there are no tasks upon which to wait
            Task.CompletedTask :
            new WhenAllPromise(tasks);

So Task.CompletedTask is indeed internal, but it is exposed by calling WhenAll() with no arguments.

The newest version of .Net (v4.6) is adding just that, a built-in Task.CompletedTask:

Task completedTask = Task.CompletedTask;

That property is implemented as a no-lock singleton so you would almost always be using the same completed task.

Task<T> is implicitly convertable to Task, so just get a completed Task<T> (with any T and any value) and use that. You can use something like this to hide the fact that an actual result is there, somewhere.

private static Task completedTask = Task.FromResult(false);
public static Task CompletedTask()
{
    return completedTask;
}

Note that since we aren't exposing the result, and the task is always completed, we can cache a single task and reuse it.

If you're using .NET 4.0 and don't have FromResult then you can create your own using TaskCompletionSource:

public static Task<T> FromResult<T>(T value)
{
    var tcs = new TaskCompletionSource<T>();
    tcs.SetResult(value);
    return tcs.Task;
}