In order to lock your computer, you need to use the following two APIs:

[DllImport("user32.dll")]        public static extern void LockWorkStation();              [DllImport("user32.dll")]        public static extern int ExitWindowsEx(int uFlags, int dwReason); 

You should implement it as follows:

// Lock computer      void btnLock_Click(object sender, System.EventArgs e)      {         LockWorkStation();      }// Log Off      void btnLogOff_Click(object sender, System.EventArgs e)      {         if(DialogResult.Yes==MessageBox.Show("Log Off?","Log Off", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2))         ExitWindowsEx(0, 0);      }   // Shutdown      void btnShutdown_Click(object sender, System.EventArgs e)      {         if(DialogResult.Yes==MessageBox.Show("Shutdown?","Shutdown", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2))         ExitWindowsEx(1, 0 );