URL Rewriter Managed Fusion strip a QueryString

I'm Using Managed Fusion URL Rewriter, so far it has been awesome. Now I just need to make sure a query string is passed in a URL from a proxy web开发者_运维知识库site. I Need to pass CODE=777 on every call so I have the code below:

RewriteRule ^(.*)  http://www.somewebsite.com/$1?CODE=777[P, QSA,I,L]

However I need to strip the query string before hand. So I used the code below:

RewriteCond %{QUERY_STRING}  ^CODE=([0-9]+)+$
RewriteRule ^(.*) http://www.somewebsite.com/$1? [P, QSA]
RewriteRule ^(.*)  http://www.somewebsite.com/$1?CODE=777[P, QSA,I,L]

But it fails when I redirect with additional querystring such as below

http://proxyserver.com?othercode=something

Any Ideas how to strip a particular querystring without removing all query strings paramters?

I downloaded the source code from Managed Fusion and added the project to my solution.

I searched and found this bit of code below that appends the QueryString. I updated the code by adding a new method to remove the querystring CODE and affix a FIXED one.

I also added a Setting to control the FIXED Value assigned to the CODE.

  private Uri AppendQueryString(Uri substituedUrl, Uri existingUrl)
    {
        string append = existingUrl.Query.TrimStart('?');

        if (!String.IsNullOrEmpty(append))
        {
            UriBuilder builder = new UriBuilder(substituedUrl);

            if (String.IsNullOrEmpty(builder.Query) || builder.Query == "?")
                builder.Query = append;
            else
                builder.Query = builder.Query.TrimStart('?') + "&" + append;

            return AppendFixedQueryString(builder.Uri, existingUrl);
        }

        return AppendFixedQueryString(substituedUrl, existingUrl);
    }

ADDED NEW METHOD

    private Uri AppendFixedQueryString(Uri substituedUrl, Uri existingUrl)
    {
        string append = string.Format("CODE={0}", Settings.Default.CODE);


        if (!String.IsNullOrEmpty(append))
        {
            UriBuilder builder = new UriBuilder(substituedUrl);
            builder.Query = Regex.Replace(builder.Query, @"CODE=([0-9]+)", String.Empty);
            if (String.IsNullOrEmpty(builder.Query) || builder.Query == "?")
                builder.Query = append;
            else
                builder.Query = builder.Query.TrimStart('?').Trim("&".ToCharArray()) + "&" + append;
            return builder.Uri;
        }
        return substituedUrl;
    }

With this modification. even if the user explictly types CODE=123 at the URL it will just be ignored and a fixed CODE value will be assigned