public partial class Form1 : Form
 {

//设置文本内容的消息

private const int WM_SETTEXT = 0x000C;

 //鼠标点击消息
const int BM_CLICK = 0x00F5;[DllImport("user32.dll")]

private static extern IntPtr FindWindow(
  string lpClassName,
  string lpWindowName);

[DllImport("User32.dll")]
private static extern IntPtr FindWindowEx(
  IntPtr hwndParent,
  IntPtr hwndChildAfter,
  string lpszClass,
string lpszWindows);
[DllImport("User32.dll")]
private static extern Int32 SendMessage(
  IntPtr hWnd,       
  int Msg,       
  IntPtr wParam,      
StringBuilder lParam);

 void button1_Click(object sender, EventArgs e)
{
  // 返回写字板主窗口句柄
  IntPtr hWnd = FindWindow("Notepad", "Untitled - Notepad");
  if (!hWnd.Equals(IntPtr.Zero))
  {
    //返回写字板编辑窗口句柄
    IntPtr edithWnd = FindWindowEx(hWnd, IntPtr.Zero, "Edit", null);
    if (!edithWnd.Equals(IntPtr.Zero))
      // 发送WM_SETTEXT 消息： "Hello World!"
      SendMessage(edithWnd, WM_SETTEXT, IntPtr.Zero, new StringBuilder("Hello World!"));
  }
}
}

另：

 Message msg = Message.Create(hwnd_button, BM_CLICK, new IntPtr(0), new IntPtr(0));
//点击hwnd_button句柄对应的按钮
 PostMessage(msg.HWnd, msg.Msg, msg.WParam, msg.LParam);