Creating Classes & ActiveX Controls

If you want a property to be read-only at design time, then you can check the Ambient.UserMode property. It returns False when at design time, and True when at run-time.

Dim m_vFile As Variant
Public Property Get vFile() As Variant
    '// we are happy for the client to read vFile at both run time and design time
    vFile = m_vFile
End Property

Public Property Let vFile(ByVal New_vFile As Variant)
    '// only allowed to set value at run time
    '// raise an error if we try to set it at design time.
    If Ambient.UserMode = False Then Err.Raise 382
    m_vFile = New_vFile
    PropertyChanged "vFile"
End Property

So, you can see that if you only want to be able to read and set a property at design time, then you insert
If Ambient.UserMode = False Then Err.Raise 387
into both the Property Let and Property Get procedures, and likewise, if you only want to be able to read and set a property at run time, then you insert
If Ambient.UserMode Then Err.Raise 387
into both the Property Let and Property Get procedures.