C# 5.0 Programmer's Reference

Parameter Declarations ❘ 85
Because the DoubleIt method doesn’t require that its parameter be initialized, the DoubleTest
method does not initialize its value variable. The DoubleIt method sets the parameter’s value and
displays it. Because the parameter is declared out, the value returns to the calling code, so in the
DoubleTest method value becomes 50.
The following text shows the result.
DoubleIt: 50
DoubleTest: 50
Unusual Circumstances and Exceptions
Even if you know how to pass arguments by value, by reference, and for output, there are situations
that can be confusing. Recall that some variables are value types and others are reference types.
When you pass a value type variable by value, the method receives a copy of the value and all is as
you would expect.
If you pass a reference variable by value, the method receives a copy of the reference, not a copy of
the reference’s data. That means the method cannot change the reference (because it was passed by
value) but it can change the data associated with the reference.
For example, consider the following method.
private void SetCity(Person person)
{
person.City = "Bugsville";
person.Zip = 12345;
}
This method takes a Person parameter passed by value, changes the object’s City and Zip values,
and ends.
If Person is a class, it is a reference type so the method receives a copy of the reference. That means
the parameter refers to the same object as the argument in the calling code, so any changes to the
object’s properties are also changes to the original object.
In the calling code, the Person object hasn’t changed but its City and Zip values have.
Now consider the following version of the SetCity method.
private void SetCity(Person person)
{
person = new Person() { City = "Programmeria", Zip = 54321 };
}
This version sets the person parameter to a new Person object. Because the parameter was passed
by reference, however, the calling code’s Person object isn’t changed. You can make the calling code
receive the new value by declaring the parameter ref or out.
A similar phenomenon occurs when you pass arrays into a method. If the array is passed by value,
the method can change its values, but if it sets the parameter equal to a new array, the change does
not return to the calling code. If the array parameter is declared ref or out, the method can set the
parameter equal to a new array, and the change returns to the calling code.
Chapter 6 has more to say about methods.
www.it-ebooks.info