Ref – Out of C#

The ref keyword causes argument passed by reference.

The effect is that any changes is made to the parameter in the method will be reflected in that variable when control passes back to the calling method.

To use a ref parameter, both the method definition and the calling method must explicitly use the ref keyword.

An argument passed to to a ref parameter must first be initialized. This differs it from out whose argument need not be explicitly initialize before being passed.

Both ref and out are treated differently at runtime, but treated the same at compilation. Therefore, methods can't be overloaded, if one method takes a ref keyword and the other takes an out argument.

Here is short video tutorial describing Ref-Out: