ref out

They're pretty much the same - the only difference is that a variable you pass as an

out

parameter doesn't need to be initialized but passing it as a

ref

parameter it has to be set to something.

int x;
Foo(out x); // OK

int y;
Foo(ref y); // Error: y should be initialized before calling the method

Ref

parameters are for data that might be modified,

out

parameters are for data that's an additional output for the function (eg

int.TryParse

) that are already using the return value for something.