How to use IsError function in VBA?

In VBA (Visual Basic for Applications), the `IsError` function is used to check if an expression results in an error. This can be particularly useful when you are performing operations that might fail under certain conditions, such as looking up a value in a range or performing a calculation with potentially invalid input.

Here’s how you can use the `IsError` function in VBA:

Open the Visual Basic for Applications editor, by pressing `ALT+F11` in Excel, or by navigating to the Developer tab and clicking on “Visual Basic” if you have the Developer tab enabled.
Insert a new module or open an existing one in the VBA editor.
Write a VBA subroutine or function within which you would like to use `IsError`.

Example usage:

Sub CheckForError()
    Dim result As Variant
    Dim isThereAnError As Boolean
    
    ' Perform some calculation that might result in an error
    On Error Resume Next ' This is to prevent VBA from stopping execution if an error is encountered
    result = Application.WorksheetFunction.VLookup("NonExistentValue", Range("A1:B10"), 2, False)
    On Error GoTo 0 ' Resume normal error handling
    
    ' Use IsError to check if result contains an error
    isThereAnError = IsError(result)
    
    If isThereAnError Then
        MsgBox "There was an error in the calculation!"
    Else
        MsgBox "The result of the calculation is " & result
    End If
End Sub

In this example, the `IsError` function is used to check if the `VLookup` function returned an error because it didn’t find the value “NonExistentValue” in the specified range. If `IsError` returns `True`, a message box is displayed indicating that there was an error.

Note that the use of `On Error Resume Next` and `On Error GoTo 0` around the potentially error-prone code is good practice to prevent the VBA macro from halting execution upon encountering an error. Instead, it allows the `IsError` function to be used to handle the error as part of the normal flow of execution.

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