How to call Visual C/C++ implemented DLL functions in C# - Simple DLLImport

Managed world is beautiful, I have all classes I want in FrameWork.. But what if I want call some unmanaged code?

For instance, I have DLL written in C++, and want use it from C#.

Let's look some code. Our DLL exports some function, in CDecl convention, that sums two integers:

extern "C" __declspec(dllexport) __cdecl int sum(int a,int b);

And, of course, we want reuse this code in C#. We must recall, that it is no "direct" way to call unmanaged code, but we must inform the compiler, what we want to call, how, and where is needed code located.
 

[DllImport("TestDll.dll", EntryPoint="sum",
ExactSpelling=false,CallingConvention=CallingConvention.Cdecl)]
static extern int sum(int a,int b);

and now we can call it like normal C# function.

x=5;
y=7;
z=sum(x,y);			// z will receive 12