var queryString = QueryHelpers.ParseQuery("?param1=value");
  // queryString.GetType() --> typeof(Dictionary)

  // Adding a parameter
  queryString.Add("param2", "my value"); // Easy so far.

  // Adding a second parameter with the same name
  queryString.Add("param2", "my other value"); // ArgumentException!
  // --> An item with the same key has already been added