Check if an application is idle for a time period and lock it

You can use these functions

• LockWorkStation
• GetLastInputInfo

see this code, you must add a timer to your form, and set this.timer1.Enabled = true;

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication9
{
  internal struct LASTINPUTINFO
  {
    public uint cbSize;    
    public uint dwTime;
  }

  public partial class Form1 : Form
  {

    [DllImport("User32.dll")]
    public static extern bool LockWorkStation();
    [DllImport("User32.dll")]
    private static extern bool GetLastInputInfo(ref LASTINPUTINFO Dummy);
    [DllImport("Kernel32.dll")]
    private static extern uint GetLastError();

public static uint GetIdleTime()
{
  LASTINPUTINFO LastUserAction = new LASTINPUTINFO();
  LastUserAction.cbSize = (uint)System.Runtime.InteropServices.Marshal.SizeOf(LastUserAction);
  GetLastInputInfo(ref LastUserAction);
  return ((uint)Environment.TickCount - LastUserAction.dwTime);
}

public static long GetTickCount()
{
  return Environment.TickCount;
}

public static long GetLastInputTime()
{
  LASTINPUTINFO LastUserAction = new LASTINPUTINFO();
  LastUserAction.cbSize = (uint)System.Runtime.InteropServices.Marshal.SizeOf(LastUserAction);
  if (!GetLastInputInfo(ref LastUserAction))
  {
    throw new Exception(GetLastError().ToString());
  }

  return LastUserAction.dwTime;
}

    public Form1()
    {
      InitializeComponent();
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
      if (GetIdleTime() > 10000)  //10 secs, Time to wait before locking
        LockWorkStation();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
      timer1.Start();
    }
  }
}