ASP.Net WebForms malformed querystring, ? rather than & - asp.net

Question

In one of our webforms apps we have external links coming to the site where there are 2 querystring parameters, but the second param is also preceded by a ?.
Normally, your querystring will only have one ?, which is at the beginning just before the first param, and any subsequent params are preceded by &. For example:
www.somesite.com?param1=a&param2=b <---- this is properly formed
www.somesite.com?param1=a?param2=b <---- this is malformed
Yes, I know that param values can contain question marks, and it is best to escape them, but we don't have that issue.
These urls are coming from an external source and we can't do anything about them right now, but we do need to parse the querystrings properly.
With the above malformed url, Request.QueryString["param1"] yields:
a?param2=b
But if the url were properly formed it would yield:
a
Also if properly formed, Request.QueryString["param2"] would yield:
b
How best to handle such a situation, if you are unable to fix the source of the problem? I might add that the url comes to the site urlencoded.

Answer 1

This is the solution that I have come up with. Just fix the querystring and redirect back. In the Page_Load, I call this ProcessQS method, and have added the fix qs code to it:
private bool ProcessQS()
{
var param1 = Request.QueryString["param1"];
if (string.IsNullOrWhiteSpace(param1))
return false;
// Workaround for external links that have ? instead of & for querystring parameter beyond the first.
// In this case, id should be preceded by &, this handles those urls that have a ? preceding id.
if (param1.Contains("?param2="))
{
var qs = Request.ServerVariables["QUERY_STRING"];
qs = HttpUtility.UrlDecode(qs);
Response.Redirect($"~/somepage.aspx?{qs.Replace("?param2=", "&param2=")}", true);
}
return true;
}