Sub Procedures and Functions

Procedures are made up of series of Visual Basic statements that, when called, are executed. After the call is finished, control returns to the statement that called the procedure. In this way, procedures make it simple for you to package your code into discrete units. Ideally, each Visual Basic procedure should handle one—and only one—task, to make this easy to remember. You can pass data to procedures and the code in the procedure can work on that data. As mentioned above, there are two types of procedures in Visual Basic .NET: Sub procedures and functions. Sub procedures do not return a value, while functions do.

Let's take a look at creating a Sub procedure first. We've already placed all our executable code in the Sub procedure named Main in the previous chapter, so this will be easy to do:

Module Module1
    Sub Main()
        System.Console.WriteLine("Hello from Visual Basic")
    End Sub
End Module

When this console application starts, control is transferred to the Main Sub procedure automatically, and the code in it runs. However, we can create our own Sub procedures as well, as below where I'm creating a Sub procedure named DisplayMessage to display the same message the above code does:

Module Module1
    Sub Main()
    End Sub

    Sub DisplayMessage()
        System.Console.WriteLine("Hello from Visual Basic")
    End Sub
End Module

To execute the code in DisplayMessage, you must call that Sub procedure, which looks like this:

Module Module1
    Sub Main()
        DisplayMessage()
    End Sub

    Sub DisplayMessage()
        System.Console.WriteLine("Hello from Visual Basic")
    End Sub
End Module

This produces the same results as before, displaying the message "Hello from Visual Basic"; when you call DisplayMessage, the code in that Sub procedure is executed. Note the parentheses following DisplayMessage above; you use these to enclose data you pass to the procedure, which are called arguments. For example, to pass to DisplayMessage the text string we want to display, you can indicate that it accepts a text-string argument, like this:

Module Module1
    Sub Main()
    End Sub

    Sub DisplayMessage(ByVal strText As String)
        ⋮
    End Sub
End Module

Here, the keyword ByVal indicates that the text string is passed by value, which means a copy of the string is passed. This is the default in VB .NET. The other possibility is ByRef, which means that the argument will be passed by reference. When you pass a variable by reference (which was the default in VB6 and earlier), the location of the variable is passed to the procedure, which means you have direct access to that variable back in the calling code. Changing the value in that variable (as by assigning it a new value like this: intArgument1 = 5) actually changes its value back in the code that called the procedure. In this way, if you pass variables by reference (but not by value) to a procedure, the code in that procedure can change the value in those variables.

Now that I've given the argument passed to DisplayMessage a name (strText), I can refer to that argument by name in the body of DisplayMessage:

Module Module1
    Sub Main()
    End Sub

    Sub DisplayMessage(ByVal strText As String)
        System.Console.WriteLine(strText)
    End Sub
End Module

And I can pass data to DisplayMessage when I call it (this string, "Hello from Visual Basic", will be stored in strText in DisplayMessage):

Module Module1
    Sub Main()
        DisplayMessage("Hello from Visual Basic")
    End Sub

    Sub DisplayMessage(ByVal strText As String)
        System.Console.WriteLine(strText)
    End Sub
End Module

This code displays our message as before.

You can also create functions, which return values. For example, I might create a function named Addem that accepts two integer arguments and returns their sum. Declaring a function is much like declaring a Sub procedure, except that you use the keyword Function instead of Sub, and specify the return type of the function like this (note that you separate multiple arguments in the declaration of a procedure with commas):

Module Module1
    Sub Main()
    End Sub

    Function Addem(ByVal int1 As Integer, ByVal int2 As Integer) As Long
        Return int1 + int2
    End Function
End Module

You return a value from a function with the Return statement, as I have here, where I'm returning the sum of the two arguments passed to us. You also can avoid using the Return statement if you simply assign a value to the name of a function, as in this example, where the Always5 function always returns a value of 5:

   Private Sub Form1_Load(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles MyBase.Load
       MsgBox(Always5())
   End Sub

   Private Function Always5() As Integer
       Always5 = 5
   End Function

When you call a function by using its name and an argument list enclosed in parentheses, that name is replaced by the value returned by the function. For example, the call Addem(2, 2) is replaced by the value 4, as in this code:

Module Module1
    Sub Main()
        Dim intValue As Integer = 2
        System.Console.WriteLine("{0}+{1}={2}", _
            intValue, intValue, Addem(intValue, intValue))
    End Sub

    Function Addem(ByVal int1 As Integer, ByVal int2 As Integer) As Long
        Return int1 + int2
    End Function
End Module

When you run this code, you see this result:

2+2=4