Sometimes it is necessary to use an assembly that was not created in .Net. This usually involves doing some low level things that are not native to C# or .Net and require you to import an existing assembly, perhaps a Win32 assembly, or to write your code in C++ and then import that into C#.

Let’s say for example you wanted to call the Win32 MessageBeep method to output an audible prompt to your user when they type something incorrectly. Yes, I know that would be annoying but this is just an example.

First we need to call the Win32 assembly with P/Invoke like this.

[DllImport("User32.dll")]
static extern Boolean MessageBeep(UInt32 beepType);

Now this imports the MessageBeep functionality so we can use it in C#. Now all you do is call it like a normal Method passing the beep type as a parameter.

MessageBeep(0);

It is that simple. P/Invoke gives use some nice flexibility to leverage existing Win32 API functions that may be missing from C# or reuse unmanaged code we have written for other projects.