Delegate usage with callback from unmanaged code

Question

This is from one of the C# books. What I don't understand is that for this to be a callback, shouldn't the EnumWindow call to PrintWindow be made from user32.dll. Maybe, I am not understanding delegates or callback properly. Please help me to understand.

Callbacks from Unmanaged Code

The P/Invoke layer does its best to present a natural programming model on both sides of the boundary, mapping between relevant constructs where possible. Since C# can not only call out to C functions but also can be called back from the C functions (via function pointers), the P/Invoke layer maps unmanaged function pointers into the nearest equivalent in C#, which is delegates.

As an example, you can enumerate all top-level window handles with this method in User32.dll:

      BOOL EnumWindows (WNDENUMPROC lpEnumFunc, LPARAM lParam);

     WNDENUMPROC is a callback that gets fired with the handle of each window in sequence (or until the callback returns false). Here is its definition:

BOOL CALLBACK EnumWindowsProc (HWND hwnd, LPARAM lParam);

To use this, we declare a delegate with a matching signature, and then pass a delegate instance to the external method:

  using System;
 using System.Runtime.InteropServices;

 class CallbackFun
{
    delegate bool EnumWindowsCallback (IntPtr hWnd, IntPtr lParam);

   [DllImport("user32.dll")]
   static extern int EnumWindows (EnumWindowsCallback hWnd, IntPtr lParam);

   static bool PrintWindow (IntPtr hWnd, IntPtr lParam)
   {
        Console.WriteLine (hWnd.ToInt64());
        return true;
    }

    static void Main()
    {
        EnumWindows (PrintWindow, IntPtr.Zero);
    }
 }