In VS, the difference between using and "Add reference" is vsusing.

In VS, the difference between using and "Add reference" is vsusing.

Reference indicates that you can use this Assembly, and add a using to facilitate your use of this Assembly, without the need to write the full name.

For example:
After referencing System. Windows. Form, you can use the MessageBox. Show () method;
But before you use using System. Windows. Form, you must use:
System. Windows. Form. MessageBox. Show ()
After you add using System. Windows. Form in front of the code, you can easily use it directly:
MessageBox. Show ()
That is, the usage of "class. Method"
Of course, I use static methods here. Other methods need to be instantiated first.
Anything you want to use must come from an assembly you reference,A reference is a reference of an assembly to another assembly;

A References B, indicating that A can work normally only when the B Assembly exists. Possible reasons include: resource, code, and so on. In short, if the B assembly cannot be found, assembly A cannot be started.
Using indicates a reference to a namespace in a code unit, so that your code uses the name in the namespace without a full name.

A little more, in fact, this principle is very simple, isn't it.