The Request.QueryString property is declared as System.Collections.Specialized.NameValueCollection but the internal implementation is really a derived (undocumented) class called System.Web.HttpValueCollection.
You can access each query string parameter using the index operator [], but you cannot modify the parameter values or add or delete a parameter from the collection:
var q = Request.QueryString;
// => read-only System.Web.HttpValueCollection
if (q["param"] == null)
q.Add("param", "some-value");
This code will throw a System.NotSupportedException.
Why do you want to manipulate the query string anyway? Say you have a request and you want to add/change/remove a parameter of the query string, and then redirect to the modified URL. Of course, you want to make sure that each parameter is in the URL exactly once. Request.QueryString[] provides a simply mechanism to access and modify the parameters and their values.
To get a modifiable NameValueCollection from the current request, we need the helper method System.Web.HttpUtility.ParseQueryString (which again returns an HttpValueCollection):
var q = System.Web.HttpUtility.ParseQueryString(
Request.QueryString.ToString());
if (q["param"] == null)
q.Add("param", "some-value");
var r = Request.Path + "?" + q.ToString();
Response.Redirect(r);
To redirect from the current request, we take the request’s Path and concatenate the collection’s ToString() result. The HttpValueCollection.ToString() adds ‘&’ and ‘=’ between parameters and their values (as opposed to NamedValueCollection.ToString() which just returns the class name).
