The Optional Keyword

In addition to the keywords ByVal and ByRef, you could also use the Optional keyword to define a variable as not required, but passable.

In a function statement this might look like this

Public Function MyFunction(ByRef MyInteger As Integer, _ Optional MyNote As String) As String

By doing as such, you would specify to the program that you don't have to insert the MyNote variable for the function to actually perform an operation. You can leave it to the user to provide specific additional elements for your statement to evaluate.

The important thing to think about here is that ALL arguments passed as Optional MUST be last in the statement definition. Every argument following the first optional argument must also be an optional argument. All arguments can be optional, but they can not be entered in mixed order.

To follow the principles of defaults in Visual Basic, I would say it is good coding practice to keep ByRef first, since it is the default state, then followed by ByVal arguments and at last the Optional arguments.

As you see form the end above, the final definition is that the function MyFunction will return a value MyFunction As String as long you enter a correct and accepted value for the MyInteger argument.

Default Values

A benefit of using the Optional keyword is the use of default values. This can save on defining constants with yet another benefit. Constants can not change values in the application. However providing specific default value to some statements may well act as interchangeable constants.

Declaring constants in VB is not much different from variables however constants are declared in the Declaration section of a form code or module.

Variables are declared inside Statements, while constants are declared prior to any statements.

Const MyConstant As String = "A string constant"

In the same way as this declaration, you can define default values in the definition part of a statement, but only for optional arguments.

To define the constant above as a default value only to be used in the following function instead of defining it as a constant you could write;

Public Function MyFunction(ByRef MyInteger As Integer, _ Optional MyNote As String = "A String Constant") As String

Also note the benefit with this action. You can now go in and change the default value should you choose to do so. This would be impossible had you instead called a constant inside the statement.