ID may be of type string, int, Guid,custom type, etc. Setter is protected to allow unit tests to set this property via reflection and to allow domain objects more flexibility in setting this for those objects with assigned IDs.

'Declaration
 
Public Overridable ReadOnly Property Id As TId

'Usage
 
Dim instance As DomainObject(Of TId)
Dim value As TId
 
value = instance.Id

public virtual TId Id {get;}