How can I send a whole .Net project as attachment via gmail?

Another answer is to encrypt the zip file or rar file. If you use winrar you can setup a password and choose "encrypt file names". This will bypass the gmail filter.

If you zip up everything in your project except the /bin and the /obj folders, you should be okay sending the project to them. When they get the project, they simply need to compile it again to get the executable, but all the code will already be there.