Receiveing null when using SPFarm.Local

Receiveing null when using SPFarm.Local

I have got the same code running well as an exe application. But when I
created a class library project and insert the same code. The SPFarm.Local
returned null. I have set the Platform target to x64 and I logged in the
server as Administrator.
here is the environment list: windows server 2008 r2, visural studio 2010,
sharepoint 2010