Why does C# encounter this error as to the CSC file?

These are not errors in the code but errors in the references of your project. Check that the references are indeed located where they point.

It is possible that when doing your SVN update, some source files have been moved but you didn't update your .csproj containing the references to these files.

This question occurred a long time ago, but for others that view this, make sure that the c# files are on your hard drive. Sometimes the Solution Explorer shows that they are there, but they aren't.

This can also happen if you renamed your startup class, but didn't update the startup object in your project settings.