How can I remove item from querystring in asp.net using c#?

Here is a simple way. Reflector is not needed.

    public static string GetQueryStringWithOutParameter(string parameter)
    {
        var nameValueCollection = System.Web.HttpUtility.ParseQueryString(HttpContext.Current.Request.QueryString.ToString());
        nameValueCollection.Remove(parameter);
        string url = HttpContext.Current.Request.Path + "?" + nameValueCollection;

        return url;
    }

Here QueryString.ToString() is required because Request.QueryString collection is read only.

If it's the HttpRequest.QueryString then you can copy the collection into a writable collection and have your way with it.

NameValueCollection filtered = new NameValueCollection(request.QueryString);
filtered.Remove("Language");