C# edit values while stepping through code in .net debugger

Question

In a C# 2008 desktop application while using the .net debugger, I can sometimes change the values of some fields in the locals window. However there are other times I can not change the value. 

The following is a couple of examples of circumstances I would like to know how to change the value:
1. A .net object is not created and the value is set to null. I would like to be able to change some of the values in the object that was not created.
2. A string [] table contains not elements. I would like to be able to add rows to the table that originally contained no rows.

Can you tell me how to edit values in the above circumstances while using the .net debugger?