使用 IsMissing:

If IsMissing(arg) Then
    MsgBox "Parameter arg not passed"
End If

但是，如果我没记错的话，这在为参数提供默认值时不起作用，并且在任何情况下都会使使用默认参数变得相当多余.

However, if I remember correctly, this doesn’t work when giving a default for the argument, and in any case it makes using the default argument rather redundant.