How do i...fix this problem so that my test case would pass

You are trying to use an object that has had Dispose called on it because it wa no longer wanted. That releases all memory and resources allocated to the object and returns it to the Heap pool. Subsequent attempts to use the actual object by any means will cause an error.

The simplest way to generate the error is to create an instance of a class inside a using block, and keep a copy of it:

private MyClass theInstanceInQuestion;
...
   using (MyClass instance = new MyClass)
      {
      ... do something with the instance here ...
      theInstanceInQuestion = instance;
      } // At this point, instance is Disposed.

We can't tell you how to fix this: we have no access to your code or the test cases you are using - but somehow you have maintained a reference to an object that is Disposed and are trying to use it again!