This is because references are copied at compile time.
Your TestCaseClass app has a reference to the assembly myBO.DLL. This DLL reference is copied into the bin\debug folder of your TestCaseClass project. But if you make changes to myBO and recompile it, the new DLL does not get copied into the bin\debug of your TestCaseClass app (it will get copied if you recompile TestCaseClass, which is the behaviour you are currently seeing).
If you don't want to recompile your TestCaseClass, you can copy the new myBO.DLL into it's bin\debug folder instead.
~~Bonnie
>Hi,
>Thank you for reply.
>
>I have TestCaseClass called TestMyBO in project TestMyApp. I have a class called myBO in project MyApp.
>
>While my test failed, I fixed my code in myBO class (without change its interface) and recompile it. I re-run my test again, it will still fail unless I recompile my TestCaseClass.
>
>Why?
>
>Thank you