C# 8.0 in a Nutshell

Chapter 25. Native and COM Interoperability

This chapter describes how to integrate with native (unmanaged) Dynamic-Link Libraries (DLLs) and Component Object Model (COM) components. Unless otherwise stated, the types mentioned in this chapter exist in either the System or the System.Runtime.InteropServices namespace.

Calling into Native DLLs

P/Invoke, short for Platform Invocation Services, allows you to access functions, structs, and callbacks in unmanaged DLLs (shared libraries on Unix).

For example, consider the MessageBox function, defined in the Windows DLL user32.dll as follows:

int MessageBox (HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType);

You can call this function directly by declaring a static method of the same name, applying the extern keyword, and adding the DllImport attribute:

using System;
using System.Runtime.InteropServices;

class MsgBoxTest
{
  [DllImport("user32.dll")]
  static extern int MessageBox (IntPtr hWnd, string text, string caption,
                                int type);
  public static void Main()
  {
    MessageBox (IntPtr.Zero,
                "Please do not press this again.", "Attention", 0);
  }
}

The MessageBox classes in the System.Windows and System.Windows.Forms namespaces themselves call similar unmanaged methods.

Here’s a DllImport example for Ubuntu Linux:

[DllImport("libc")]
public static extern uint getuid();

public static void PrintUserID()
{
  Console.WriteLine ($"User ID: {getuid()}");
}

The CLR includes a marshaler that knows how to convert parameters and return values ...