I needed to construct a PNG file on the fly, so that its content was constructed at the moment it was downloaded. This technique will also work with GIF and JPEG files. This proved easier than I expected in .NET. The graphic is constructed entirely virtually, and is never saved to disk. In fact, there is no file with the graphic name at all.

Step 1, You need to tell IIS that the expected file type needs to be processed by the .NET framework. This has to occur in the IIS Manager dialogs. You may need to get your ISP to perform this task for you.

Right click on the web site in IIS manager and select Properties.
Click on the Home Directory tab.
Click on Configuration button. In the Application Configuration dialog, you are going to set the mapping for .PNG files (or GIF or JPEG) to run through the .NET processing. The easiest way is to...
Select the line in the dialog for an existing extension that is mapped to aspnet_isapi.dll, such as .config and click Edit.
Select and Copy (using Ctrl+C) the path in the Executable textbox from this entry and select Cancel.
Press the Add button.
Paste (using Ctrl+V) the path that you copied.
In the extension textbox add your extension, such as .png.
In the Verbs section, select the Limit To option and put GET, HEAD into the textbox.
Press OK and close all the dialogs

Step 2, Next you have to tell IIS that it should call a piece of code when a file is requested. In the web.config file for the web site, add the following section as a child of <system.web>:

<httpHandlers>
    <add verb="GET" path="mygraphic.png" type="MyNameSpace.MyImage, MyAssemblyName" /> 
</httpHandlers>

This tells .NET that when mygraphic.png is requested that it should instantiate the MyNameSpace.MyImage class.

Step 3, Create the MyImage class. The MyImage class is responsible for drawing the graphic. Add the following class to the web site:

using System; 
using System.Collections;
using System.Diagnostics;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Web; 

namespace MyNameSpace
{ 
    public class MyImage: IHttpHandler
    {
        public void ProcessRequest(HttpContext context)
        {
            Bitmap bmp;
            Graphics gph; 
            Brush brushFill;
            System.IO.MemoryStream ms; 

            context.Response.BufferOutput = true; 
            context.Response.Clear();
            context.Response.ClearHeaders(); 
            context.Response.ClearContent();
            context.Response.ContentType = "image/png";
            context.Response.Expires = 0; 
            context.Response.Cache.SetCacheability(HttpCacheability.NoCache); 
            context.Response.Cache.SetNoServerCaching(); 
            context.Response.Cache.SetNoStore(); 
            context.Response.Cache.SetMaxAge(System.TimeSpan.Zero); 

            bmp = new Bitmap(100, 100, PixelFormat.Format24bppRgb);
            gph = Graphics.FromImage(bmp);
            brushFill = new SolidBrush(Color.Blue);
            gph.FillRectangle(brushFill, 1, 1, 50, 50); 
            ms = new System.IO.MemoryStream();
            bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Png);
            Byte[] abyte = ms.ToArray(); 
            context.Response.OutputStream.Write(abyte, 0, abyte.Length); 
            context.Response.End();
            bmp.Dispose();
            gph.Dispose();
        } 

        public Boolean IsReusable
        { 
            //To enable pooling, return true here.
            //This keeps the handler in memory.
            get
            {
                return false;
            }
        }
    }
}

Step 4, Create a web page that uses the graphic. Put the following line in the web page:

<img src="mygraphic.png" />

When the page is rendered, you will have a 100 x 100 pixel black square with a 50 x 50 pixel blue square in the upper left hand corner. You will need to study the GDI+ features of the .NET framework to modify the drawing. But that's beyond the scope of what I wanted to cover here.

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