PromiseLike.cs

public static class PromiseLike
{
    public static Task Create(Action<TaskCompletionSource> action)
    {
        ThrowIfNull(action);
        var tcs = new TaskCompletionSource();
        action.Invoke(tcs);
        return tcs.Task;
    }

    public static Task<T> Create<T>(Action<TaskCompletionSource<T>> action)
    {
        ThrowIfNull(action);
        var tcs = new TaskCompletionSource<T>();
        action.Invoke(tcs);
        return tcs.Task;
    }
}