How do I decode a URL parameter using C#?

Have you tried HttpServerUtility.UrlDecode or HttpUtility.UrlDecode?

Try:

var myUrl = "my.aspx?val=%2Fxyz2F";
var decodeUrl = System.Uri.UnescapeDataString(myUrl);

string decodedUrl = Uri.UnescapeDataString(url)

or

string decodedUrl = HttpUtility.UrlDecode(url)

Url is not fully decoded with one call. To fully decode you can call one of this methods in a loop:

private static string DecodeUrlString(string url) {
    string newUrl;
    while ((newUrl = Uri.UnescapeDataString(url)) != url)
        url = newUrl;
    return newUrl;
}

Server.UrlDecode(xxxxxxxx)