Datasheet

A class consists of both state and behavior. State is a fancy way of referring to the fact that the class has
one or more values also known as properties associated with it. Embedded in the class defi nition are zero
or more Dim statements that create variables used to store the properties of the class. When you create an
instance of this class, you create these variables; and in most cases the class contains logic to populate them.
The logic used for this, and to carry out other actions, is the behavior . This behavior is encapsulated in
what, in the object - oriented world, are known as methods .
However, Visual Basic doesn ’ t have a “ method ” keyword. Instead, it has two other keywords that are brought
forward from Visual Basic ’ s days as a procedural language. The fi rst is
Sub . Sub , short for “ subroutine, ” and
it defi nes a block of code that carries out some action. When this block of code completes, it returns control to
the code that called it without returning a value. The following snippet shows the declaration of a Sub :
Private Sub Load(ByVal object As System.Object)
End Sub
The preceding example shows the start of a Sub called Load . For now you can ignore the word Private
at the start of this declaration; this is related to the object and is further explained in the next chapter. This
method is implemented as a Sub because it doesn ’ t return a value and accepts one parameter when it is called.
Thus, in other languages this might be considered and written explicitly as a function that returns Nothing .
The preceding method declaration for
Sub Load also includes a single parameter, object , which is declared
as being of type System.Object . The meaning of the ByVal qualifi er is explained in chapter 2, but is
related to how that value is passed to this method. The code that actually loads the object would be written
between the line declaring this method and the End Sub line.
Alternatively, a method can return a value; Visual Basic uses the keyword
Function to describe this behavior.
In Visual Basic, the only difference between a Sub and the method type Function is the return type.
The
Function declaration shown in the following sample code specifi es the return type of the function as
a Long value. A Function works just like a Sub with the exception that a Function returns a value, which
can be Nothing . This is an important distinction, because when you declare a function the compiler expects
it to include a Return statement. The Return statement is used to indicate that even though additional lines
of code may remain within a Function or Sub , those lines of code should not be executed. Instead, the
Function or Sub should end processing at the current line, and if it is in a function, the return value should
be returned. To declare a Function , you write code similar to the following:
Public Function Add(ByVal ParamArray values() As Integer) As Long
Dim result As Long = 0
'TODO: Implement this function
Return result
'What if there is more code
Return result
End Function
In the preceding example, note that after the function initializes the second line of code, there is a Return
statement. There are two Return statements in the code. However, as soon as the fi rst Return statement is
reached, none of the remaining code in this function is executed. The Return statement immediately halts
execution of a method, even from within a loop.
As shown in the preceding example, the function ’ s return value is assigned to a local variable until returned as
part of the
Return statement. For a Sub , there would be no value on the line with the Return statement, as a
Sub does not return a value when it completes. When returned, the return value is usually assigned to something
else. This is shown in the next example line of code, which calls a function to retrieve the currently active control
on the executing Windows Form:
Dim ctrl = Me.Add(1, 2)
The preceding example demonstrates a call to a function. The value returned by the function Add is a Long ,
and the code assigns this to the variable ctrl . It also demonstrates another keyword that you should be aware
of: Me . The Me keyword is how, within an object, that you can reference the current instance of that object.
Visual Basic Keywords and Syntax
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