What's the difference between posting from a html-form and using the webclient class?

When I Inspect with firebug I notice that when posting with a regular html form it sets the contenttype to application/x-www-form-urlencoded, but when using the Webclient class it doesn't. And when i use the webclient class from an silverlight app I must fetch th开发者_开发问答e data from the inputstream and make the corresponding conversion.

How can you add items to the Request.Form property with a webclient class? Is it possible?

Silverlight HTTP POST few variables, SIMPLEST example (least code)