First one is for status messages shown at the top of the page. Next comes the HtmlForm control which contains 3 HtmlInputFile controls.

Let's see what our code-behind class looks like.

iv. upload.aspx.cs
This class is the code-behind for upload.aspx page you saw above. It contains the code which handles uploading of upto 3 image files simultaneously. Different rules are applied to those 3 image files. The first one's size should be between 5 KB and 20 KB, second one's width and height should be less than 300 pixels, while there is no restriction on the size and dimensions of the third one. These restrictions have only been placed to teach you how to limit image uploading to a certain size or dimension.

Next it creates an extra thumbnail ( 150x150 pixel ) for every image uploaded.

Now create a new file as upload.aspx.cs and copy/paste following code in it:

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Imaging;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Text;
using System.Text.RegularExpressions;

namespace Stardeveloper.UploadImage
{
  public class UploadForm : System.Web.UI.Page
  {
    public const int MaxWidth = 300;
    public const int MaxHeight = 300;

    public const int MinSize = 5 * 1024;
    public const int MaxSize = 20 * 1024;

    public const int ThumbnailWidth = 150;
    public const int ThumbnailHeight = 150;

    protected HtmlGenericControl message;

    protected HtmlInputFile file;
    protected HtmlInputFile file1;
    protected HtmlInputFile file2;

    private void Page_Load(object sender, System.EventArgs e)
    {
      if(IsPostBack)
      {
        message.Attributes.Add("align", "left");

        UploadImageWithSizeRestrictions(file.PostedFile);
        UploadImageWithDimensionRestrictions(file1.PostedFile);
        UploadImageWithNoRestrictions(file2.PostedFile);
      }
      else
      {
        message.Attributes.Add("align", "center");
        AppendMessage("Select file to upload.");
      }
    }

    private void AppendMessage(string msg)
    {
      message.Controls.Add(new LiteralControl(msg));
    }

    private void DisplayImageInfo(HttpPostedFile image)
    {
      AppendMessage(String.Format("FileName: {0}<br>", image.FileName));
      AppendMessage(String.Format("FileSize: {0} KB<br>",
        image.ContentLength / 1024));

      using(Bitmap bitmap = new Bitmap(image.InputStream, false))
      {
        AppendMessage(String.Format("Width: {0}<br>", bitmap.Width));
        AppendMessage(String.Format("Height: {0}<br>", bitmap.Height));

        if(bitmap.RawFormat.Equals(ImageFormat.Bmp))
        {
          AppendMessage("ImageFormat: Bitmap Image (BMP)<br>");
        }
        if(bitmap.RawFormat.Equals(ImageFormat.Gif))
        {
          AppendMessage("ImageFormat: Graphics Interchange Format (GIF)<br>");
        }
        if(bitmap.RawFormat.Equals(ImageFormat.Jpeg))
        {
          AppendMessage("ImageFormat: Joint Photographic Experts " +
            "Group (JPEG)<br>");
        }
        if(bitmap.RawFormat.Equals(ImageFormat.Png))
        {
          AppendMessage("ImageFormat: W3C Portable Network Graphics (PNG)<br>");
        }
      }
    }

    private bool IsImage(HttpPostedFile file)
    {
      if(file != null && Regex.IsMatch(file.ContentType, "image/\\S+") &&
        file.ContentLength > 0)
      {
        DisplayImageInfo(file);
        return true;
      }

      return false;
    }

    protected void UploadImageWithSizeRestrictions(HttpPostedFile file)
    {
      if(IsImage(file))
      {
        int contentLength = file.ContentLength;
        if(contentLength > MinSize && contentLength < MaxSize)
        {
          UploadImage(file);
        }
        else
        {
          AppendMessage(String.Format("<span style=\"color:red;\">Image size " +
            "exceeding limits: {0} KB</span><br><br>",
            contentLength / 1024));
        }
      }
    }

    protected void UploadImageWithDimensionRestrictions(HttpPostedFile file)
    {
      if(IsImage(file))
      {
        using(Bitmap bitmap = new Bitmap(file.InputStream, false))
        {
          if(bitmap.Width < MaxWidth && bitmap.Height < MaxHeight)
          {
            UploadImage(file);
          }
          else
          {
            AppendMessage(String.Format("<span style=\"color:red;\">Image " +
              "dimensions exceeding limits: {0} x {1}</span><br><br>",
              bitmap.Width, bitmap.Height));
          }
        }
      }
    }

    protected void UploadImageWithNoRestrictions(HttpPostedFile file)
    {
      if(IsImage(file))
      {
        UploadImage(file);
      }
    }

    protected void UploadImage(HttpPostedFile file)
    {
      string fileName =
        file.FileName.Substring(file.FileName.LastIndexOf('\\') + 1);
      string filePath = MapPath("uploaded_images/" + fileName);

      file.SaveAs(filePath);
      AppendMessage("<span style=\"color:green;\">File Uploaded...</span><br>");

      using(System.Drawing.Image image =
        System.Drawing.Image.FromStream(file.InputStream))
      using(Bitmap bitmap = new Bitmap(image, ThumbnailWidth, ThumbnailHeight))
      {
        bitmap.Save(MapPath("uploaded_images/t__" + fileName), image.RawFormat);
        AppendMessage("<span style=\"color:green;\">Thumbnail" +
          " created...</span><br><br>");
      }
    }

    #region Web Form Designer generated code
    override protected void OnInit(EventArgs e)
    {
      //
      // CODEGEN: This call is required by the ASP.NET Web Form Designer.
      //
      InitializeComponent();
      base.OnInit(e);
    }
    
    /// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InitializeComponent()
    {    
      this.Load += new System.EventHandler(this.Page_Load);
    }
    #endregion
  }
}