How to mock an instantiated object

Can I mock an instantiated object? Here's my scenario:

public class A {
protected SomeCollectionType m_Collection;

public void AddDataMember (B dataMember) {
m_Collection.Add(dataMember);
m_Collection.SetCurrent(dataMember);
}

public bool FuncToBeTested () {
B data = m_Collection.GetCurrent();

if (data.State1) {
// do something for state 1
....
}
else if (data.State2) {
// do something for state 2
....
}
....
}
}

I only want to test FuncToBeTested(), so I wrote a test function like this:
[Test, VerifyMocks]
public void test_FuncToBeTested()
{
Mock mock = MockManager.Mock(typeof(B));

mock.ExpectGet("State1", Null);

bool bRet = m_testObject.FuncToBeTested();
Assert.That(bRet, Is.EqualTo(false));
}

I ran the test using command line command "TMockRunner.exe nunit-console.exe ModuleA.UnitTest.dll" with proper path information. What I'm getting is an error:
test_FuncToBeTested : System.NullReferenceException : Object reference not set to an instance of an object.

In the TypeMock tracer I see that State1 method is not called (0/1).

Can someone help with this code? From what I can see here, I'm guessing that Mock object actually mocks the function, regardless of which object owns that function. Is this correct?

Thanks in advance.


Richard Zhu