using System;

using System.Collections.Generic;

using System.Threading.Tasks;

using AutoFake;

var events = new List<string>();

var fake = new Fake<Calendar>();

var sut = fake.Rewrite(() => Calendar.AddSomeMinutes(new DateTime(2016, 8, 8)));

​

sut.Prepend(() => events.Add("The first line"));

sut.Prepend(() => events.Add("The line before AddMinutes(...) call"))

   .Before((DateTime date) => date.AddMinutes(Arg.IsAny<int>()));

​

sut.Append(() => events.Add("The line after new Random() call"))

   .After(() => new Random());

sut.Append(() => events.Add("The last line"));

​

sut.Execute();

Console.WriteLine(string.Join(Environment.NewLine, events));

​

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));

}