I am using a specific command in in my C# code, which works well. However, it is said to misbehave in "unmanaged" code.
What is managed or unmanaged code?
Here is some text from MSDN about unmanaged code.
Some library code needs to call into unmanaged code (for example, native code APIs, such as Win32). Because this means going outside the security perimeter for managed code, due caution is required.
Here is some other complimentary explication about Managed code:
For your problem:
I think it's because NUnit execute your code for UnitTesting and might have some part of it that is unmanaged. But I am not sure about it, so do not take this for gold. I am sure someone will be able to give you more information about it. Hope it helps!
This is a good article about the subject.
An application program that is executed within a runtime engine installed in the same machine. The application cannot run without it. The runtime environment provides the general library of software routines that the program uses and typically performs memory management. It may also provide just-in-time (JIT) conversion from source code to executable code or from an intermediate language to executable code. Java, Visual Basic and .NET's Common Language Runtime (CLR) are examples of runtime engines. (Read more)
An executable program that runs by itself. Launched from the operating system, the program calls upon and uses the software routines in the operating system, but does not require another software system to be used. Assembly language programs that have been assembled into machine language and C/C++ programs compiled into machine language for a particular platform are examples of unmanaged code.(Read more)