microsoft moles, dynamically instrument
using ClassLibrary1;
using Microsoft.Moles.Framework;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using MoleClassLibrary;
[assembly: MoledAssembly(typeof(ClassLibrary1.Class1))]

namespace TestProject1
{
    [TestClass()]
    public class Class1Test
    {
        [TestMethod()]
        [HostType("Moles")]
        public void TestStringTest()
        {
            var target = new Class1();
            var expected = "Mole value.";
            string actual;
            MClass1.AllInstances.TestString = value => expected;
            actual = target.TestString();
            Assert.AreEqual(expected, actual);
        }
    }
}