using System;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        var stringTask = Task.FromResult("sample");

        stringTask.TeeAsync(st =>

        {

            var l = st.Length;

            return Task.CompletedTask;

        }).Wait();

    }

}

​

public static class Extensions

{

    public static async Task<T> TeeAsync<T>(this T source, Func<T, Task> asyncAction)

    {       

        Console.WriteLine("first overload");

        await Task.Delay(0); // todo: do something with source

        return source;

    }

    public static async Task<T> TeeAsync<T>(this Task<T> asyncSource, Func<T, Task> asyncAction)

    {

        Console.WriteLine("second overload");

        var source = await asyncSource;

        await Task.Delay(0); // todo: do something with source

        return source;

    }

}