using System;

using System.Threading.Tasks;

using AutoFake;

var randomValue = 7;

var date = new DateTime(2016, 8, 8, hour: 0, minute: 0, second: 0);

var fake = new Fake<Calendar>();

var sut = fake.Rewrite(f => f.AddSomeMinutesAsync(date));

sut.Replace((Random r) => r.Next(1, 10)) // Arg.Is<int>(i => i == 10) is also possible

                       // r.Next(1, 11) fails with "Expected - 11, actual - 10"

    .ExpectedCalls(1) // c => c > 1 fails with "Actual value - 1"

    .Return(randomValue);

​

Console.WriteLine(await sut.Execute());

​

public class Calendar

{

    internal Task<DateTime> AddSomeMinutesAsync(DateTime date) => Task.Run(() => AddSomeMinutes(date));

    public static DateTime AddSomeMinutes(DateTime date) => date.AddMinutes(new Random().Next(1, 10));

}