Question

Take this sample code:

Class Foo
    ReadOnly name As String

    Public Sub New(name As String, dependentUpon As Foo)
        Me.name = name
        Console.Write("{0} created. ", name)
        Console.WriteLine("Dependent upon {0}.", If(dependentUpon IsNot Nothing,
                                                    dependentUpon.Name,
                                                    "nothing"))
    End Sub
End Class

Class Bar
    ReadOnly dependent As New Foo("Dependent", independent)  ' <-- !!!
    ReadOnly independent As New Foo("Independent", Nothing)
End Class

The output of New Bar() is:

Dependent created. Dependent upon nothing.
Independent created. Dependent upon nothing.

It seems fields are initialized in the same order as they appear in the source code, which (a) leads to an unexpected result, and (b) seems a little puzzling, given that one is normally not permitted to read from uninitialized variables in .NET, yet that seems to be working fine above.

I would’ve expected VB.NET to be smart enough to initialize referenced fields first, and only then those that reference it; i.e. I’d have liked to see this output instead:

Independent created. Dependent upon nothing.
Dependent created. Dependent upon Independent.

Does someone know a way how to get VB.NET to behave like that instead, without simply having to swap the declaration order of dependent and independent inside class Bar?

Answer 1

Fields are always initialized in the order they’re declared.

The restriction against accessing uninitialized variables applies only to local variables, not fields. (that would be too hard to enforce)