What is the difference between ref and out parameters?
The significant difference between the ref keyword and the out keyword is that the out keyword doesn’t require the calling code to initialize the passed arguments first.
A more subtle difference between ref and out is that out parameters must be modified in the called method, whereas ref parameters can be but don’t have to be modified

read more:

Inside C#: Chapter 2: The .NET type system - Summary
Inside C#: Chapter 3: Defining Classes - Summary
Inside C#: Chapter 4: Methods - Summary

I wonder where part 1 is??