Added reference, class is still not defined?

I have a web site project where I need to use the System.Security.Cryptography.Xml.SignedXml class. I added the Dll System.Security as a reference and imported the namespace into my project. It compiles successfully but when I debug the project I get a compilation erro开发者_运维百科r:

Compiler Error Message: BC30002: Type 'System.Security.Cryptography.Xml.SignedXml' is not defined.

What gives?

Edit: I run Windows XP Pro 64bit OS

how you can compiles successfuly and get compilation error wile debug? Try to reload your VS...