Would it be a bad deal to start with asp.net and go picking up the c # language as it progresses in ASP.NET?

No, as long as you seek to build knowledge in C # as you study. That is, if a construction appears in the code that you do not understand very well, look for the reason why it is used and in what other cases it can and can not be used.

If not, what is the main part of C # that I should focus on before going to ASP.NET?

Some parts, in fact:

• Type Discipline: C # is a strong typing language. Avoid using dynamic types in the beginning. Leave them to when the types part is mastered;
• Handling Exceptions: Exceptions are serious problems that require special handling in code. Avoid ignoring them or creating mechanisms to stifle them. Always try to know what each one means and what to do in case they appear;
• Levels of Protection: there are several, each has a very specific function. Try to focus initially on three:
  • public
  • private
  • protected
• Interfaces and Generics: In my opinion they are the second great differential of the language: the ability to define classes that do not know exactly a certain type and the contract scheme, which guarantees code security;
• Reflection : simply the most powerful feature of language, the ability to read complex class metadata and constructs;
• Extensibility: My favorite: the ability to extend a class without knowing its code.