See Also: WebHeaderCollection Members
The System.Net.WebHeaderCollection class is generally accessed through WebRequest.Headers or WebResponse.Headers. Some common headers are considered restricted and are either exposed directly by the API (such as Content-Type) or protected by the system and cannot be changed.
The restricted headers are:
Accept
Connection
Content-Length
Content-Type
Date
Expect
Host
If-Modified-Since
Range
Referer
Transfer-Encoding
User-Agent
Proxy-Connection