The answers here are good for the time the question has been asked. In the meanwhile, IMHO, dnSpy has outperformed them.
✓ Visualize as C# or VB.NET - or IL
✓ Navigation between classes - by Ctrl+Click
✓ Examination of resources - live preview
? Handling .NET since 2.0 - I could not find such an old program for testing :-)
✓ Exporting classes to source ...