Change the query string values in C#

// reflect to readonly property

PropertyInfo isreadonly = typeof(System.Collections.Specialized.NameValueCollection).GetProperty("IsReadOnly", BindingFlags.Instance | BindingFlags.NonPublic);

// make collection editable

isreadonly.SetValue(this.Request.QueryString, false, null);

this.Request.QueryString.Remove("playermode");

this.Request.QueryString.Add("playermode", "editinitialselection");

// make collection readonly again
isreadonly.SetValue(this.Request.QueryString, true, null);