using System;

using System.Web;

​

public partial class _Default : System.Web.UI.Page

{

​

    protected void Page_Load(object sender, EventArgs e)

    {

        DateTime contentModified = System.IO.File.GetLastWriteTime("C:\\image.jpg");

​

        if (IsClientCached(contentModified))

        {

            HttpContext.Current.Response.StatusCode = 304;

            HttpContext.Current.Response.SuppressContent = true;

        }

        else 

        {

            HttpContext.Current.Response.Cache.SetLastModified(contentModified);

        }

    } 

​

    private bool IsClientCached(DateTime contentModified)

    {

        string header = HttpContext.Current.Request.Headers["If-Modified-Since"];

        if (header != null)

        {

            DateTime isModifiedSince;

            if (DateTime.TryParse(header, out isModifiedSince))

            {

                return isModifiedSince > contentModified;

            }

        }

​

        return false;

    }

}