Question

I have a class that exposes an auto implemented property Enabled

Public MustInherit Class TopLevel
    Protected Property Enabled() As Boolean
End Class

In an inhertited class I cannot access _Enabled

Public Class SubClass
    Inherits TopLevel

    Public Function Foo() As string
        If Not _Enabled Then Return Nothing '<- This fails
    End SUb
End Class

But If I had not use an auto implemented property and declared my own backing-field as follows this is accessible from the subclass:

Private _Enabled as Boolean

—- EDIT —-
The abve line is incorrect – this is not possible, it was in fact Protected in the original code which allowed access from the sub class See @JonSkeet answer
—- EDIT —-

Of course I can just access Enabled from the sub class to work around this but can someone explain why this is the behaviour?

Answer 1

But If I had not use an auto implemented property and declared my own backing-field as follows this is accessible from the subclass:

Private _Enabled as Boolean

No it wouldn’t – private fields aren’t accessible from derived classes – only protected ones are. (The exception to this being nested classes, which have access to their containing class’s private members.)

From the docs for the Private modifier:

Specifies that one or more declared programming elements are accessible only from within their declaration context, including from within any contained types.

Personally I’d regard it as bad form to access the backing field of an automatically implemented property directly anyway – in C#, it’s not even available, as it’s given an unspeakable name (one which isn’t valid in C# itself). If you want the value of the property from your derived class, why don’t you just access it as a property? That would be the appropriate approach even if you weren’t using automatically implemented properties – your fields should be private.