You C# files should contain $LastChangedRevision: $ to get the SVN revision added. However, you also need to set the svn:keywords property correctly for the files in which you want that value to be expanded.
For example, the following will result in a string containing the revision.
string revision = "$LastChangedRevision: $";
The svn:keywords property should include the LastChangedRevision in its space-separated list of values.
You can make this automatic for the cs extension by editing the TortoiseSVN configuration. To do this, go to the TortoiseSVN explorer menu and select Settings.... Then on the General tab, click the Edit button next to Subversion configuration file:.
Scroll down and add a line like this one into the [auto-props] section (just an example, you may have more or less keywords and you may also set other properties):
*.cs = svn:keywords=LastChangedDate LastChangedBy LastChangedRevision Id URL;