C# unit testing with Microsoft fakes and shims

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C#

I have a very simple class.

public class MyClass
{
    private String MyString;
    private String MyStringB;

    public MyClass()
    {
        MyString = "MyString has been set.";
        MyStringB = SetMyString();
    }

    private String SetMyString()
    { return "My String has been set by the function call."; }
}

which runs a unit test accessing its private members.

[TestClass]
public class MyTestClass
{
    String DesiredOutCome = "My String has been set by the function call.";

    [TestMethod]
    public void TestMyClassConstructor()
    {
        MyClass MyObject = new MyClass();
        PrivateObject obj = new PrivateObject(MyObject);
        var MyOutCome = obj.GetFieldOrProperty("MyStringB");
        Assert.AreEqual(DesiredOutCome, MyOutCome);
    }
    }

Now i need to do a Shim to replace the dependency SetMyString().

String NewDesiredOutCome = "MyString has been set to something different.";

[TestMethod]
public void TestMyClassConstructorWithShim()
{
	using (ShimsContext.Create())
	{
		MyNameSpace.Fakes.ShimMyClass.Constructor = (@this) =>
		{
			var shim = new ShimMyClass(@this)
			{
				SetMyString = () => { return "MyString has been set to something different."; }
			};
		};

		MyClass MyObject = new MyClass();
		PrivateObject obj = new PrivateObject(MyObject);
		var MyOutCome = obj.GetFieldOrProperty("MyStringB");
		Assert.AreEqual(NewDesiredOutCome, MyOutCome );
	};
}

But this fails with
Assert.Are equal failed. Expected <MyString has been set to something different.> Actual: <(Null)>

for some reason, the Shim which should only modify the constructor call, seems to modify the whole class, removing the member variable MyStringB... i cant see any reason it should be null.

What I have tried:

I tried stepping through the debugger, cant figure out why obj.GetFieldOrProperty returns a null to a field which exists, and the other unit test proves it does.