Apart from knowing the language (c#, vb whatever) well, take a look at the underlying framework. Newcomers omit that part most of the time. Might help with lots of decisions.
- The importance of .Net framework for your program. What happens when you double click on a .Net exe? What happens at the start of a web application? Is the framework is always there or started per app?
- Spend some time with ildasm. Learn what a manifest is. What is it good for. What is Intermediate Language (IL)?
- Difference between reference and value types. Heap and stack.
- What is garbage collection? How the garbage collector is working? IDisposable pattern.
- Why generic classes are important? What was there before them?
- Static vs. instance method calls.
- Passing object references around and keeping a reference (hence preventing GC, especially important for Desktop apps with improper use of events and references)
- For desktop devs: UI thread, background threads, patterns to pass data between these two.
- Basic OO. Virtual methods, overrides, new keyword, interfaces.
