The browser would render a webpage faster if it knows the images dimensions. But

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Asked: May 30, 20262026-05-30T16:39:21+00:00 2026-05-30T16:39:21+00:00

The browser would render a webpage faster if it knows the images dimensions. But

0

The browser would render a webpage faster if it knows the images dimensions. But how can I assign the Width and Height in ASP.NET automatically?

I can do a server control extending Image that would query the image size using GDI+ and assign those values to its own Width and Height properties.

But there are several things that worries me and would like to hear opinions:

Cache: What is the best way to implement cache? I would like to cache the Height and Width values across all pages for each image.
Which method should I use to query the image size? The constructor or another method (OnInit, OnPreRender, etc, etc)?
Is it GDI+ the best option to query image size, or is there any other efficient way?
How can I query the size for remotes images?

Of course I have an idea of how to do all that I mentioned, but I wanted to hear some thoughts.

http://code.google.com/speed/page-speed/docs/rendering.html#SpecifyImageDimensions

EDIT: This is what I have so far:

[ToolboxData("<{0}:AutoSizeImage runat=server></{0}:AutoSizeImage>")]
public class AutoSizeImage : Image
{
    protected override void OnPreRender(EventArgs e)
    {
        if (Width.Value + Height.Value == 0)
        {
            if (IsLocal(ImageUrl))
            {
                using (System.Drawing.Image img = System.Drawing.Image.FromFile(Context.Server.MapPath(ImageUrl)))
                {
                    Width = img.Width;
                    Height = img.Height;
                }
            }
            else
            {
                System.Net.WebRequest request = System.Net.WebRequest.Create(ImageUrl);
                using (System.Net.WebResponse response = request.GetResponse())
                using (System.IO.Stream responseStream = response.GetResponseStream())
                using (System.Drawing.Image img = System.Drawing.Image.FromStream(responseStream))
                {
                    Width = img.Width;
                    Height = img.Height;
                }
            }
        }
        base.OnPreRender(e);
    }
    private bool IsLocal(string p)
    {
        return !(p.StartsWith("http://") || p.StartsWith("https://") || p.StartsWith("ftp://"));
    }
}

It works beautifully, but I need to implement cache now.
I want to cache the output of this control for each value of ImageUrl, that is, I want the OnPreRender method to be called once for each image. Is that possible?