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Optional Parameters (Visual Basic)

You can specify that a procedure parameter is optional and no argument has to be supplied for it when the procedure is called. Optional parameters are indicated by the Optional keyword in the procedure definition. The following rules apply:

  • Every optional parameter in the procedure definition must specify a default value.

  • The default value for an optional parameter must be a constant expression.

  • Every parameter following an optional parameter in the procedure definition must also be optional.

The following syntax shows a procedure declaration with an optional parameter:

Sub sub name(ByVal parameter1 As datatype1, Optional ByVal parameter2 As datatype2 = defaultvalue)

When you call a procedure with an optional parameter, you can choose whether to supply the argument. If you do not, the procedure uses the default value declared for that parameter.

When you omit one or more optional arguments in the argument list, you use successive commas to mark their positions. The following example call supplies the first and fourth arguments but not the second or third:

sub name(argument 1, , , argument 4)

The following example makes several calls to the MsgBox function. MsgBox has one required parameter and two optional parameters.

The first call to MsgBox supplies all three arguments in the order that MsgBox defines them. The second call supplies only the required argument. The third and fourth calls supply the first and third arguments. The third call does this by position, and the fourth call does it by name.

MsgBox("Important message", MsgBoxStyle.Critical, "MsgBox Example")
MsgBox("Just display this message.")
MsgBox("Test message", , "Title bar text")
MsgBox(Title:="Title bar text", Prompt:="Test message")

A procedure cannot detect at run time whether a given argument has been omitted or the calling code has explicitly supplied the default value. If you need to make this distinction, you can set an unlikely value as the default. The following procedure defines the optional parameter office, and tests for its default value, QJZ, to see if it has been omitted in the call:

Sub notify(ByVal company As String, Optional ByVal office As String = "QJZ")
    If office = "QJZ" Then
        Debug.WriteLine("office not supplied -- using Headquarters")
        office = "Headquarters" 
    End If 
    ' Insert code to notify headquarters or specified office. 
End Sub

If the optional parameter is a reference type such as a String, you can use Nothing as the default value, provided this is not an expected value for the argument.

Another way to define a procedure with optional parameters is to use overloading. If you have one optional parameter, you can define two overloaded versions of the procedure, one accepting the parameter and one without it. This approach becomes more complicated as the number of optional parameters increases. However, its advantage is that you can be absolutely sure whether the calling program supplied each optional argument.

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