AssignField with method level variables

Hi

I am trying to use AssignField to set the value of a variable that is declared within the method I wish to test. It is existing code and refactoring is being avoided wherever possible.

As in the simplified example below I can set a class level variable with no problems, but trying to set the method level variable fails when attempted as shown. I've made further attempts but I can't find a way to make it work, am I missing something obvious?

Thanks for your help,
Andy

public class ClassBeingTested
{
    int a = 0;

    public void MethodBeingTested()
    {
        int b = 0;
    }
}

[TestMethod]
[Isolated]
public void TestMethod()
{
    Mock ctt = MockManager.Mock<ClassBeingTested>();
    
    ctt.AssignField("a", 1);
    ctt.AssignField("b", 1);  // This line causes the error "Type ClassBeingTested does not contain a definition for b"
}