Programming Excel with VBA and .NET : Tasks in Visual Basic - Check Results

8/12/2011 9:10:07 AM

In many situations, you need to check the result of an operation to tell if it succeeded. The result-checking functions in Visual Basic let you test results before you take actions that might otherwise cause an error. Table 1 lists the result-checking functions.

Table 1. Visual Basic result-checking functions
Category	Function	Use to
Boolean tests	IsArray	Tell if a variable is an array
	IsDate	Tell if a variable contains data that can be converted to a Date
	IsEmpty	Tell if a variable has not yet been initialized
	IsError	Tell if a variable contains an Error object
	IsMissing	Tell if a ParamArray argument was omitted
	IsNull	Tell if a variable contains no valid data
	IsNumeric	Tell if a variable contains a value that can be converted to a number
	IsObject	Tell if a variable is a reference to a valid object
Type tests	TypeName	Get the name of the variable's type as a string
	TypeOf	Test the type of a variable within an If block
	VarType	Get the variable's type as a VbVarType constant

Most of these tests are used with variables that were declared as Variant or Object data types. Those types of variables can contain many different kinds of data, so it is often necessary to test what the variable contains before proceeding in code.

There are several common uses of this in Excel. The first is ActiveSheet.property, which may refer to a Worksheet, Chart, or other object:

    Sub ChangeSheets( )
        Select Case TypeName(ActiveSheet)
        Case "Worksheet"
            If ActiveSheet.Index < Worksheets.Count Then
                Worksheets(ActiveSheet.Index + 1).Activate
            Else
                Worksheets(1).Activate
            End If
        Case "Chart"
            If ActiveSheet.Index < Charts.Count Then
                Charts(ActiveSheet.Index + 1).Activate
            Else
                Charts(1).Activate
            End If
        Case Else
            Debug.Print TypeName(ActiveSheet), ActiveSheet.Name
        End Select
    End Sub

The preceding code uses a Select statement to perform different actions based on the TypeName of the active sheet. You usually combine TypeName with SelectTypeName is also handy for checking whether or not an optional argument has been omitted: when there are more than two possibilities as shown in the preceding block.

    Public Sub Reformat(Optional ws As Worksheet)
        ' Check if argument was omitted.
        If TypeName(ws) = "Nothing" Then
            ' Check the type of the active sheet.
            If TypeName(ActiveSheet) = "Worksheet" Then
                ' Format the active worksheet.
                Set ws = ActiveSheet
            Else
                ' You can't reformat nonworksheets.
                MsgBox "Select a worksheet and try again."
                Exit Sub
            End If
        End If
        Dim rng As Range
        ' Get the cells with data in them.
        Set rng = ws.UsedRange
        ' Apply AutoFormat
        rng.AutoFormat xlRangeAutoFormatSimple
    End Sub

In the preceding code, the choices are either/or: if the argument is omitted, check the active sheet; if that sheet is a worksheet, use it. Alternately, you can use the TypeOf keyword within an If statement; however, TypeOf can't test if the variable is Nothing. To do that, you need to use either TypeName or the Is operator, as shown by this different version of the preceding code:

    Public Sub Reformat2(Optional ws As Worksheet)
        ' Check if argument was omitted.
        If ws Is Nothing Then
            ' Check the type of the active sheet.
            If TypeOf ActiveSheet Is Worksheet Then
                ' Format the active worksheet.
                Set ws = ActiveSheet
            Else
                ' You can't reformat nonworksheets.
                MsgBox "Select a worksheet and try again."
                Exit Sub
            End If
        End If
        Dim rng As Range
        ' Get the cells with data in them.
        Set rng = ws.UsedRange
        ' Apply AutoFormat
        rng.AutoFormat xlRangeAutoFormatSimple
    End Sub

Reformat and Reformat2 are equivalent. I tend to use the TypeName test rather than TypeOf or Is because it lets me use a consistent test for all types of objects, but that's really just a personal preference.

The IsNumeric and IsDate functions are useful when receiving data from a user. Rather than returning specific information about the type of the variable, they let you know if the data in the variable can be converted to those types. For instance, the following code checks the value entered in an InputBox to determine the type of entry the user made:

    Sub CheckEntry( )
        Dim var As String, msg As String
        var = InputBox("Enter a number, word, or date.")
        ' Use Boolean tests to check an entry.
        If IsNumeric(var) Then
            msg = "number."
        ElseIf IsDate(var) Then
            msg = "date."
        ElseIf var = "" Then
            msg = "empty."
        Else
            msg = "string."
        End If
        Debug.Print "Entry is a " & msg
    End Sub