I think this question reflects a fact: that you have not done the basic reading about, and experimenting with, C# that anyone need to do when starting to study the language.
You need to get a good book like Charles Petzold's "Dot Net Zero" [
^] (free), and read about the basic data Types in .NET.
You need to understand that "declaring" an instance of a Type in C# and associating that instance with a "name" ... which is what you do when you create any variable ... will have different "default" behaviors depending on the Type:
int anInt; // anInt automatically set to value #0
string aString; // null
string[] aStrArray; // null
It is good practice to always pay attention to whether or not the variables you declare have been initialized.
While the Microsoft documentation for Type int ('int is an alias for Sytem.Type.Int32) is in error when they say that you cannot use a variable of that Type (or any simple Numeric Type) without initializing it; yes, you can use it.
You need to study Type String in depth, and understand it's unique aspects (relative to the Heap and Stack internal components of .NET).
Remember that everyone here was once a "beginner," and had to go through learning the "basics" in the same way one has to learn fundamental arithmetic operations to progress in learning more advanced mathematics.