Call into Unmanaged dlls from C#

Overview 

• PInvoke is used to invoke unmanaged dlls
  • kernel32.dll
    • process loading
    • context switching
    • file, memory i/o
  • user32.dll
    • windows, menus, dialog boxes, icons, etc.
  • GDI32
    • drawing capabilities

Declare a method in c# to access unmanaged function

• Declare this method as both
  • static
  • extern
• Apply attribute System.Runtime.InteropServices.DllImportAttribute to specify
  • dll file
  • name of the unmanaged function

• Examples
  
[DllImport("kernel32.dll", EntryPoint="GetPrivateProfileString")]
private static extern int GetPrivateProfileString(string lpAppName,
string lpKeyName, string lpDefault, StringBuilder lpReturnedString,
int nSize, string lpFileName);

  
[DllImport("kernel32.dll", EntryPoint="WritePrivateProfileString")]
private static extern bool WritePrivateProfileString(string lpAppName,
string lpKeyName, string lpString, string lpFileName);

Call unmanaged function that uses callbacks

• Declare a delegate with the required signature for the callback
• Define call back function to be passed
• Pass the callback to the function
• Example
  
// The signature for the callback method.
public delegate void OnXMConnect();
public delegate void OnXMDisconnect();
// The call back methods
private static void OnXMConn(){
Console.WriteLine("Connected");
}
private static void OnXMDisconn(){
Console.WriteLine("DisConnected");
}// Pass the callbacks to unmanaged function
private static void TestXM(){
USB_RegisterCallbacks(OnXMConn, OnXMDisconn);
}

Pass Struct as parameter

• Define a class to represent C struct
• Annotate with [StructLayout(LayoutKind.Sequential)]
• Declare struct members as class member variables
• Annotate variables with [MarshalAs…] if needed
• Example
  
  // Define struct as a class
  [StructLayout(LayoutKind.Sequential)]
  public class OSVersionInfo
  {
  public int dwOSVersionInfoSize;
  public int dwMajorVersion;
  public int dwMinorVersion;
  public int dwBuildNumber;
  public int dwPlatformId;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]
  public string szCSDVersion;
  }// Use the class
  [DllImport("kernel32.dll")]
  public static extern bool GetVersionEx([In, Out] OSVersionInfo osvi);private static void TestStruct()
  {
  OSVersionInfo osvi = new OSVersionInfo();
  osvi.dwOSVersionInfoSize = Marshal.SizeOf(osvi);GetVersionEx(osvi);

  Console.WriteLine("Major version: {0}", osvi.dwMajorVersion);
  Console.WriteLine("Minor version: {0}", osvi.dwMinorVersion);
  }

// if RECT is a class, not struct

[DllImport("user32.dll")]
public static extern int GetWindowRect(int hwnd, [MarshalAs(UnmanagedType.LPStruct)] RECT rc);