Question

I have recently worked with vb.net and got a problem of a factory pattern with generic. I have already solved it though, I don’t know why it doesn’t work in vb.net. I have several years experience of c#, and I sometimes write codes like a following in c# and it works without any problems.

public class GenericTest
{
    public static void Test()
    {
        var test = Factory<TestItem>.GetTest(ProcessType.Derived);
        test.Do();
    }
}

public class TestItem
{
    public int Property1 { get; set; }
}

public class TestBase<T> where T: class
{
    public virtual void Do()
    {
        Console.WriteLine(typeof(T).ToString());
    }
}

public class TestDerived : TestBase<TestItem>
{
    public override void Do()
    {
        Console.WriteLine(typeof(TestItem).ToString());
    }
}

public class Factory<T> where T:class
{
    public static TestBase<T> GetTest(ProcessType processType)
    {
        switch (processType)
        {
            case ProcessType.Derived :
                return new TestDerived() as TestBase<T>;
            default:
                return new TestBase<T>();
        }
    }
}

public enum ProcessType
{
    Base,
    Derived
}

but when I ported the above codes to vb.net, the compiler gives me an error.

Public Class GenericTest
    Public Shared Sub Test()
        Dim test = Factory(Of TestItem).GetTest(ProcessType.Derived)
        test.DoIt()
    End Sub
End Class    

Public Class TestItem
    Public Property Property1 As Integer
End Class

Public Class TestBase(Of T As Class)
    Public Overridable Sub DoIt()
        Console.WriteLine(GetType(T).ToString())
    End Sub
End Class

Public Class TestDerived
    Inherits TestBase(Of TestItem)

    Public Overloads Sub DoIt()
        Console.WriteLine(GetType(TestItem).ToString())
    End Sub
End Class

Public Class Factory(Of T As Class)
    Public Shared Function GetTest(processType As ProcessType) As TestBase(Of T)
        Select Case processType
            Case VB_Sandbox.ProcessType.Derived
                ' This line gives me a compile error saying 'cannot convert TestDerived to TestBase(Of T)'
                Return DirectCast(New TestDerived(), TestBase(Of T))
            Case Else
                Return New TestBase(Of T)
        End Select
    End Function
End Class

Public Enum ProcessType
    Base
    Derived
End Enum

So, I introduced a following interface to make everything okay and that actually solved my problem.

Public Class TestBase(Of T As Class)
    Implements ITest
    Public Overridable Sub DoIt() Implements ITest.DoIt
        Console.WriteLine(GetType(T).ToString())
    End Sub
End Class

Public Class TestDerived
    Inherits TestBase(Of TestItem)

    Public Overloads Sub DoIt()
        Console.WriteLine(GetType(TestItem).ToString())
    End Sub
End Class

Public Class Factory(Of T As Class)
    Public Shared Function GetTest(processType As ProcessType) As ITest
        Select Case processType
            Case VB_Sandbox.ProcessType.Derived
                Return DirectCast(New TestDerived(), ITest)
            Case Else
                Return New TestBase(Of T)
        End Select
    End Function
End Class

Public Interface ITest
    Sub DoIt()
End Interface

Is this just a limitation of vb.net?

Answer 1

The main reason is that DirectCast in VB is much more like the C# (cast) operator than the as operator. The C# code works because as will just return null when TestDerived isn’t a TestBase<T> (which is any time T isn’t TestItem). DirectCast will actually fail if TestDerived isn’t a TestBase(Of T), and so you see the error.

You can get similar functionality in VB by using the Typeof Is construct, or using TryCast().

Alternatively, you could have TestDerived actually inherit from TestBase<T>, (thus making TestDerived<T>) which would make everything work without casting in both languages.