C# Null Syntax - How To Set A Field To Be Nullable

I remember being told that you can add some syntax to the end of a statement that will do something if the field is null. Pretty vague, but I cant seem to find anything about it via google.

I definitely isnt how to set a field to be nullable. I know that.

Here is an example of what I want.

int? field1 = 0;

field1 = int.parse(string2); (where string2 is a passed value, that could be null)

I thought there was something you could put at ?? if it is a null, such as:

field1 = int.parse(string2) ??;

Or do I have to check every if every string2 is null before making that statement.