Request.Querystring shows HTTP Error 404.19 - Not Found

Question

In C# and ASP.NET, I've been sending a value to another page via Request.Querystring for years, but just recently I've come against a page error that I cannot explain.

If I send a string that has

0X

in it, then the page crashes with an error

HTTP Error 404.19 - Not Found

Also I cannot get a try / catch to work, to tell me more about the issue.

I can send a string with 0X locally in VS2022 and it traces through fine, but when pushed to the live server code, it gets this issue.

So, I knocked up the simplest of .aspx pages called QSRequest.aspx to prove the issue, but I can find no reason for it.

Here's an example that works:

QSRequest.aspx?SearchValue=KDZ2X

and the same code but doesn't work:

QSRequest.aspx?SearchValue=KDZ0X

My Page_Load code:

protected void Page_Load(object sender, EventArgs e)
{
    // Get Search box value
    string SearchValue = Request.QueryString["SearchValue"];

    // Check if SearchValue is not null or empty
    if (!string.IsNullOrEmpty(SearchValue))
    {
        // display the SearchValue
        Label1.Text += "SearchValue: " + SearchValue;
    }
    else
    {
        Label1.Text += "<br/>No search value provided.";
    }
 }

On doing some research, it looks like something to do with being a protected / 'system' keyword or IIS Request Filtering (if so is there a good guide to change the settings anywhere / the correct place to edit my web.config file).

Greatly appreciate ideas if I need to encode / decode it as a work around, modify the web.config. Thanks.

If you have not seen it yet, have a look: stackoverflow.com/questions/59714161/…. — PM 77-1
– PM 77-1, Commented Jul 18 at 21:12
Thank you for the link, I've decided to just use a Session instead, but it does look like it is something to do with filtering, slightly annoying that the error message of 404 isn't more intuitive. — FlashTrev
– FlashTrev, Commented Jul 19 at 8:06
I cannot replicate the issue. It could indeed be a setting in IIS and/or Web.Config. Or do you have a URL Rewrite active that redirects to the wrong page in the background — VDWWD
– VDWWD, Commented Jul 19 at 10:08

Zari Habibi · Accepted Answer · 2025-08-27 14:42:26Z

0

you have to check QueryString key befor assigning it to a variable.

the correct form is:

if (Request.QueryString["SearchValue"] != null)
{
     Label1.Text += "SearchValue: " + Request.QueryString["SearchValue"];
}
else
{
    Label1.Text += "<br/>No search value provided.";
}

answered Aug 27 at 14:42

Zari Habibi

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