Default properties in VB.Net?

Question

In the early days of .Net, I believe there was an attribute you could decorate a class with to specify a default property.

According to some articles I've found, this appears to have been yanked from the framework at some point because it was a little confusing, and I can see how that is the case.

Still, is there another way to get the functionality it provided?

It looked something like this:

<DefaultProperty("Value")> _  
Public Class GenericStat
    ...
    Public Property Value() As Integer
        ...
    End Property
    ...
End Class

This allowed you to do: Response.Write(MyObject) instead of Response.Write(MyObject.Value) ...This is not a terribly clunky example, but in some complex OO contexts it gets a little hideous. Please let me know if there is a better way.

Note: I am not looking for the Default keyword, which can only be used on properties that take a parameter.

Thanks in advance!

Answer 1

There is a DefaultProperty attribute so your example should be correct, but this seems to be for components which are used in the Windows Forms desinger.

Answer 2

You could override the ToString method to output Value as a string so that when you do Response.Write(MyObject), you get the same effect.

    Public Overrides Function ToString() As String
        Return Me.Value.ToString 
    End Function

[EDIT] Now that I understand it better, why not just provide a way to get directly at the values of the contained objects.

Public Class MyClass
    Private m_Stats(100) As Stats  ' or some other collection'

    Public Property StatValue(ByVal stat_number As Integer) As _
        Integer
        Get
            Return m_Stats(stat_number)
        End Get
        Set(ByVal Value As Integer)
            m_Stats(stat_number) = Value
        End Set
    End Property
End Class

Answer 3

To answer my own question, operator overloading seemed to be an interesting solution here.

In the end, it wasn't a good fit.

I ended up having to add in about 17 1-line methods, which means lots of room for bugs. More important is that you can't overlaod the assignment operator; the overload for = is for equality testing only.

So even with this, I can't say:

Dim x as Integer = MyObject.Stats(Stat.Health) ...In this example it pulls the object, but does not convert it to an integer, so of course the result is an exception.

What I really need is a good old fashioned default property, but I think those days are over.

Answer 4

Well, the .NET framework does have a notion of a default member. Key ingredients are the DefaultMemberAttribute class and Type.GetDefaultMembers(). In VB.NET, specifying the default member is part of the language syntax:

  Public Class Sample
    Private mValue As Integer
    Default Public ReadOnly Property Test(ByVal index As Integer) As Integer
      Get
        Return index
      End Get
    End Property
  End Class

Use it like this:

  Sub Main()
    Dim s As New Sample
    Console.WriteLine(s(42))
    Console.ReadLine()
  End Sub

The compiler implements this by emitting [DefaultMember] automatically. This however has a restriction, the property must have an index argument, specifically to avoid the syntax ambiguity. This restriction is not enforced when specifying the attribute explicitly:

  <System.Reflection.DefaultMember("AnotherTest")> _
  Public Class Sample
    Public ReadOnly Property AnotherTest() As Integer
      Get
        Return 42
      End Get
    End Property
  End Class

But that default member would only be accessible as a default by a language that allows such syntax. For which I don't know an example.

C# has the exact same rules, but doesn't allow the same kind of flexibility. You've probably seen the indexer before:

  public class Sample {
    public int this[int index] {
      get { return index; }
    }
  }

This code also makes the compiler output the [DefaultMember] attribute. The named property in that attribute is "Item". And that's why you see the indexer documented and indexed in the MSDN Library as "Item".

Answer 5

No, that was explicitly removed from VB 7.

When you have a long chain of default properties, knowing exactly what will be returned is hard. When both b and c have a Foo method, does a.Foo(1) mean a.b.Foo(1) or a.b.c.Foo(1)?

The real kicker was Set. By dropping default properties, they were also able to drop the Set keyword for object assignment.

Answer 6

In this example it pulls the object, but does not convert it to an integer.

Brian, I don't see why your desired effect cannot be achieved using a Widening Operator CType. The code you showed us can be made to work. However, beware of implicit conversions. They're generally not a good idea.