How to build a query string for a URL in C#?

Question

A common task when calling web resources from a code is building a query string to including all the necessary parameters. While by all means no rocket science, there are so

Answer 1

I wrote some extension methods that I have found very useful when working with QueryStrings. Often I want to start with the current QueryString and modify before using it. Something like,

var res = Request.QueryString.Duplicate()
  .ChangeField("field1", "somevalue")
  .ChangeField("field2", "only if following is true", true)
  .ChangeField("id", id, id>0)
  .WriteLocalPathWithQuery(Request.Url)); //Uses context to write the path

For more and the source: http://www.charlesrcook.com/archive/2008/07/23/c-extension-methods-for-asp.net-query-string-operations.aspx

It's basic, but I like the style.