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Exercise solution:
Passing references as ref parameters

The C# ref parameter other in the method DayDifference will be an alternative name of the private instance variable dateOfBirth in an object of class Person. The year of this date of birth is incremented. This is the same effect as with the call-by-value parameter.

If the parameter other is a ref parameter it is important to notice what happens, if we assign to this parameter. We do not, however, in the concrete program. If we did, we could arrange that the private instance variable dateOfBirth in the Person object becomes a reference to a new Date.

Here is a tabular overview of the two exercises and the programs on the corresponding slides:

	class	struct
by value	Like call by reference in C. The parameter `other` is a reference to another date. We can mutate this date, but we cannot change the value of the actual parameter `dateOfBirth` by assigning to `other`. The birthday can be mutated.	A struct `dateOfBirth` is copied to `other`. The year of the copied `Date` is changed. It does not affect the dateOfBirth. This is pure value semantics. Value types (structs) and call by value fit nicely together. The birthday is protected from mutation.
by C# ref	The formal parameter `other` becomes an alias of the actual parameter `dateOfBirth`. We can therefore mutate this date, as above. If we assign to `other` we do, in reality, assign to `dateOfBirth`. This observation is the real difference between this case and the case above. The birthday can be mutated.	As above, the formal parameter `other` becomes an alias of the actual parameter `dateOfBirth`. If we mutate the year of `other`, we change the year of `dateOfBirth`. If we assign to other we will (by copying and value semantics) change the `dateOfBirth`. The birthday can be mutated.

Exercise solution: Passing references as ref parameters

Exercise solution:
Passing references as ref parameters