What .net files should be excluded from source control?

Depends on the project, but I've got the following for a Silverlight + WPF project in my .gitignore:

# Visual Studio left-overs
*.suo        # 'user' settings like 'which file is open in Visual Studio'
*.ncb        # Used for debugging
*.user
*.ccscc      # Used for versioning
*.cache

# Editor left-overs
*~           # (x)emacs
*.bak        # Windows related
\#*\#        # (x)emacs
*.orig       # Own usage

# Compiled files
*/bin/
*/obj/
*/Obj/       # git is case sensitive
*/Generated_Code/
PrecompiledWeb
*/ClientBin
# Windows left-overs
Thumbs.db    # Having images in the source tree generates those files in Explorer

However, the '.suo' is somewhat problematic: it also contains 'user' settings which should have been project settings, like the startup page for a Silverlight application.

The best and only way is to iteratively add the files to exclude. If you're using git, using git-gui to quickly and interactively see the list of files which you've forgotten to exclude. Adapt .gitignore and refresh in git-gui. Iterate until the files left over are the ones you typed in.

Some types of files are not quite clear up front. Be sure you understand all the files you check in. For example, for the RIA services in our Silverlight project we had an authentication database generated by Visual Studio which contained 2 accounts and resulted in a hefty 10Mb .MDB database file(!). Once we understood where it came from, changing it to an SQL dump reduced the size to a (still hefty) 500Kb. Constantly (re)checking prior to the checkin itself is always required, so no list is definite.

It really depends on your build system. Check in the minimum files you need to run a full build.

Generally, this means you exclude everything except your csproj, and *.cs files. You can probably check in your .sln file if you want.