Microsoft defines two broad categories for source code. Managed code compiles into bytecode and is then executed by a virtual machine. Unmanaged code is compiled directly into machine code. All C# code is managed but it is possible to call external unmanaged code. This rule finds extern methods that are implemented by unmanaged code. Managed code has many advantages over unmanaged code such as built in memory management performed by the virtual machine and the ability to run compiled programs on a wider variety of architectures.