Programming Excel with VBA and .NET

Write-Once/Write-Only Properties

These types of properties are rarely needed and I thought about omitting them, but in the interest of being thorough, I decided to include some discussion here. It’s easy to create a write-only property—just omit the Get procedure—but it’s hard to even think of a situation in which that’s useful to anyone...maybe setting a password or something:

    Private m_Password As String

    ' Write-only property, rarely used.
    Property Let Password(value As String)
        m_Password = value
    End Property

Because there is only a Let procedure and m_Password is Private, users can set the Password property but they can’t get it. That might also be useful for database connection strings that can include username and password information that you should keep secure.

Write-once properties are somewhat more useful because they can represent information used to initialize an object. Once they are initialized, you usually don’t want those settings to change, so a write-once property makes sense.

Write-once properties check to see if they have been previously set, and if they have, they raise an error:

    Private m_Connection As String

    ' Write-once property, use to initialize object settings.
    Property Let ConnectionString(value As String)
        If m_Connection <> "" Then
            Err.Raise 2001, "ConnectionString", "Property already set"
        Else
            m_Connection = value
        End If
    End Property

In this case, ConnectionString is both write-once and write-only since I don’t want others to see the setting once it is ...