Introduction

A function is used to perform an assignment. The main difference between a sub procedure and a function is that, after carrying its assignment, a function gives back a result. We also say that a function "returns a value". To distinguish both, there is a different syntax you use for a function.

To create a function, you use the Function keyword followed by a name and parentheses. Unlike a sub procedure, because a function returns a value, you must specify the type of value the function will produce. To give this information, on the right side of the closing parentheses, you can type the As keyword, followed by a data type. To indicate where a function stops, type End Function. Based on this, the minimum syntax used to create a function is:

AccessModifier(s) Function FunctionName() As DataType
    
End Function

As seen for a sub procedure, a function can have an access modifier. The rules are the same as we described for a sub procedure.

The Function keyword is required.

The name of a function follows the same rules and suggestions we reviewed for sub procedures.

The As keyword may be required (in the next sections, we will review the alternatives to the As DataType expression).

The DataType factor indicates the type of value that the function will return. If the function will produce a word or a group of words, you can create it as String. The other data types are also valid in the contexts we reviewed them. Here is an example:

Function GetFullName() As String
        
End Function

As done with the variables, you can also use a type character as the return type of a function and omit the As DataType expression. The type character is typed on the right side of the function name and before the opening parenthesis. An example would be GetFullname$(). As with the variables, you must use the appropriate type character for the function:

Here is an example:

Function GetFullName$()

End Function

As mentioned already, the section between the Function and the End Function lines is the body of the function. It is used to describe what the function does. As done on a sub procedure, one of the actions you can perform in a function is to declare a (local) variable and use it as you see fit. Here is an example:

Function CallMe() As String
    Dim Salute As String
    Salute = "You can call me Al"
End Function

After performing an assignment in a function, to indicate the value it returns, somewhere after the assignment and before the End Function line, you can type the name of the function, followed by the = sign, followed by the value the function returns. Here is an example in which a function returns a name:

Function GetFullName$()
        Dim FirstName As String, LastName As String

        FirstName = InputBox("Enter First Name: ")
        LastName = InputBox("Enter Last Name: ")

        GetFullName = LastName & ", " & FirstName
End Function

Alternatively, instead of using the name of the function to indicate the value it returns, you can type Return, followed by the value or the expression that the function returns. Based on this, the above function could also be created as follows:

Function GetFullName$()
    Dim FirstName As String, LastName As String

    FirstName = InputBox("Enter First Name: ")
    LastName = InputBox("Enter Last Name: ")

    Return LastName & ", " & FirstName
End Function

You can also use some local variables in the function to perform an assignment and then assign their result to the name of the function.

1. To add a project to the current solution, on the main menu, click File -> Add -> New Project
2. Set the Name to Rectangular and click OK
3. In the Solution Explorer, under Rectangular, right-click Module1 and click Rename
4. Type Rectangle.vb and press Enter
5. To create a function, change the file as follows:
    

   Public Module Rectangle Private Function CalculatePerimeter() As Double Dim dblLength As Double Dim dblWidth As Double dblLength = InputBox("Enter Rectangle Length: ") dblWidth = InputBox("Enter Rectangle Width: ") CalculatePerimeter = (dblLength + dblWidth) * 2 End Function Public Sub Main() End Sub End Module

6. Save all

As done for the sub procedure, in order to use a function in your program, you must call it. Like a sub procedure, to call a function, you can simply type its name in the desired section of the program. Here is an example:

Sub Main()
    CallMe
End Sub

Since the primary purpose of a function is to return a value, to better take advantage of such a value, you can assign the name of a function to a variable in the section where you are calling the function. Here is an example:

Module Exercise

    Function GetFullName$()
        Dim FirstName As String, LastName As String

        FirstName = InputBox("Enter First Name: ")
        LastName = InputBox("Enter Last Name: ")

        Return LastName & ", " & FirstName
    End Function

    Friend Sub Main()
        Dim FullName$

        FullName = GetFullName()
        MsgBox(FullName)
    End Sub

End Module

Here is an example of running this program: