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using System.Runtime.InteropServices;

[DllImport ( "user32.dll", EntryPoint = "FindWindow", SetLastError = true )]
private static extern IntPtr FindWindow( string lpClassName, string lpWindowName );

[DllImport ( "user32.dll", EntryPoint = "FindWindowEx", SetLastError = true )]
private static extern IntPtr FindWindowEx( IntPtr hwndParent, uint hwndChildAfter, string lpszClass, string lpszWindow );

[DllImport ( "user32.dll", EntryPoint = "SendMessage", SetLastError = true, CharSet = CharSet.Auto )]
private static extern int SendMessage( IntPtr hwnd, uint wMsg, int wParam, int lParam );

[DllImport ( "user32.dll", EntryPoint = "SetForegroundWindow", SetLastError = true )]
private static extern void SetForegroundWindow( IntPtr hwnd );

private void button1_Click( object sender, EventArgs e )
{
    const uint BM_CLICK = 0xF5; //鼠标点击的消息，对于各种消息的数值，大家还是得去API手册

    IntPtr hwndCalc = FindWindow ( null, "计算器" ); //查找计算器的句柄

    if ( hwndCalc != IntPtr.Zero )
    {
    IntPtr hwndThree = FindWindowEx ( hwndCalc, 0, null, "3" ); //获取按钮3 的句柄

    IntPtr hwndPlus = FindWindowEx ( hwndCalc, 0, null, "+" );  //获取按钮 + 的句柄
    IntPtr hwndTwo = FindWindowEx ( hwndCalc, 0, null, "2" );  //获取按钮2 的句柄
    IntPtr hwndEqual = FindWindowEx ( hwndCalc, 0, null, "=" ); //获取按钮= 的句柄
    SetForegroundWindow ( hwndCalc );    //将计算器设为当前活动窗口
    System.Threading.Thread.Sleep ( 2000 );   //暂停2秒让你看到效果
    SendMessage ( hwndThree, BM_CLICK, 0, 0 );
    System.Threading.Thread.Sleep ( 2000 );   //暂停2秒让你看到效果
    SendMessage ( hwndPlus, BM_CLICK, 0, 0 );
    System.Threading.Thread.Sleep ( 2000 );   //暂停2秒让你看到效果
    SendMessage ( hwndTwo, BM_CLICK, 0, 0 );
    System.Threading.Thread.Sleep ( 2000 );   //暂停2秒让你看到效果
    SendMessage ( hwndEqual, BM_CLICK, 0, 0 );

    System.Threading.Thread.Sleep ( 2000 );
    MessageBox.Show ("你看到结果了吗？");
    }
    else
    {
    MessageBox.Show ("没有启动 [计算器]");
    }
}