Mastering Visual Basic .NET

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Chapter 8 BUILDING CUSTOM CLASSES
The statements of Listing 8.9 will produce the following output:
They’re equal
They’re equal
The two variables are initially equal. No surprise. After modifying one of the obj2 variable’s properties,
however, they’re still equal, because obj2 points to obj1. Every time we change obj2, obj1 also
changes. That’s because we’ve made obj1 point to obj2. They both point to the same object (or
instance of the class), and you can access this object through either class.
Comment out the line that sets obj2 equal to obj1. Now, they’re not equal, even if you set all their
fields to the same values. They don’t reference the same object, and it’s possible to set their properties
differently.
In the following section, we’ll add an Equals method that checks for value equality (as opposed to
reference equality) by comparing the values of the properties of the two instances.
Customizing Default Members
As you recall, when you created the Minimal class for the first time, before adding any code, the
class already exposed a few members—the default members, such as the ToString method (which
returns the name of the class) and the Equals method (which compares two objects for reference
equality). You can provide your custom implementation for these members; this is what we’re going
to do in this section. You already know how to do this. Your custom ToString method must be
implemented as a public function, and it must override the default implementation. The implementation
of a custom ToString method is shown next:
Public Overrides Function ToString() As String
Return “The infamous Minimal class”
End Function
It’s that simple. The Overrides keyword tells the compiler that this implementation overwrites
the default implementation of the class. Ours is a very simple method, but you can return any string
you can build in the function. For example, you can incorporate the value of the BDate property in
the string:
Return(“MINIMAL: “ & tBDate.ToString)
tBDate is a local variable in the class’s module, and you can use its value in any way you see fit in your
code. The value of the local variable tBDate is the current value of the BDate property of the current
instance of the class.
When called through different variables, the ToString method will report different values. Let’s
say you’ve created and initialized two instances of the Minimal class with the following statements:
Dim obj1 As New Minimal()
Obj1.Bdate = #1/1/1963#
Dim obj2 As New Minimal()
Obj2.Bdate = #12/31/1950#
Console.WriteLine(obj1.ToString)
Console.WriteLine(obj2.ToString)