C# passes parameters by value unless you specify that you want it differently. If the parameter type is a struct, its value is copied, otherwise the reference to the object is copied. The same goes for return values.
You can modify this behavior using the ref or out modifier, which must be specified both in the method declaration and in the method call. Both change the behavior for that parameter to pass-by-reference. That means you can no longer pass in more complex expressions. The difference between ref and out is that when passing a variable to a ref parameter, it must have been initialized already, while a variable passed to an out parameter doesn’t have to be initialized. In the method, the out parameter is treated as uninitialized variable and must be assigned a value before returning.