Create Indexer in VB.NET which can be used from C#

Question

Can I create a class in VB.NET which can be used from C# like that:

myObject.Objects[index].Prop = 1234;

Sure I could create a property which returns an array. But the requirement is that the index is 1-based, not 0-based, so this method has to map the indices somehow:

I was trying to make it like that, but C# told me I cannot call this directly:

   Public ReadOnly Property Objects(ByVal index As Integer) As ObjectData
        Get
            If (index = 0) Then
                Throw New ArgumentOutOfRangeException()
            End If
            Return parrObjectData(index)
        End Get
    End Property

EDIT Sorry if I was a bit unclear:

C# only allows my to call this method like

myObject.get_Objects(index).Prop = 1234

but not

myObject.Objects[index].Prop = 1234;

this is what I want to achieve.

Nation

Default is the keyword that you're missing. Brian's answer has got you covered. – Riddell

You are asking for an indexed property, which is a feature not directly available in C#. – Homeward

Answer 1

You can fake named indexers in C# using a struct with a default indexer:

public class ObjectData
{
}

public class MyClass
{
    private List<ObjectData> _objects=new List<ObjectData>();
    public ObjectsIndexer Objects{get{return new ObjectsIndexer(this);}}

    public struct ObjectsIndexer
    {
        private MyClass _instance;

        internal ObjectsIndexer(MyClass instance)
        {
            _instance=instance;
        }

        public ObjectData this[int index]
        {
            get
            {
                return _instance._objects[index-1];
            }
        }
    }
}

void Main()
{
        MyClass cls=new MyClass();
        ObjectData data=cls.Objects[1];
}

If that's a good idea is a different question.

Stickney

Important to note that this is a good solution only if your ObjectData object is a class. If it's a struct, you won't be able to store a reference to it in ObjectsIndexer (at best you can get a copy). This makes using this pattern to wrap unsafe structs with fixed buffers problematic, because you can't use fixed buffers in a class and you can't have a ObjectsIndexer pointing to a struct. – Beason

Answer 2

The syntax is:

Default Public ReadOnly Property Item(ByVal index as Integer) As ObjectData
  Get
    If (index = 0) Then
      Throw New ArgumentOutOfRangeException()
    End If
    Return parrObjectData(index)
  End Get
End Property

The Default keyword is the magic that creates the indexer. Unfortunately C# does not support named indexers. You are going to have to create a custom collection wrapper and return that instead.

Public ReadOnly Property Objects As ICollection(Of ObjectData)
  Get
    Return New CollectionWrapper(parrObjectData)
  End Get
End Property

Where the CollectionWrapper might would look like this:

Private Class CollectionWrapper
  Implements ICollection(Of ObjectData)

  Private m_Collection As ICollection(Of ObjectData)

  Public Sub New(ByVal collection As ICollection(Of ObjectData))
    m_Collection = collection
  End Sub

  Default Public ReadOnly Property Item(ByVal index as Integer) As ObjectData
    Get
      If (index = 0) Then
        Throw New ArgumentOutOfRangeException()
      End If
      Return m_Collection(index)
    End Get
  End Property

End Class

Orchard

And if I want the property to have a name other then Item? – Nation

You forgot the "property" keyword. – Farahfarand

Answer 3

C# doesn't support the declaration of named indexed properties (although you can create indexers), but you can access indexed properties declared in other languages (like VB) by calling the setter or getter explicitly (get_MyProperty/set_MyProperty)

Spevek

Answer 4

Why not make use of the 0 based indexing but give the illusion to the coder that it is 1 based?

ie

Return parrObjectData(index-1)

Desman

How should the method signature look like? – Nation

The same as it is, this should be the only line that should change. Apart from removing the (index = 0) if statement block – Desman

But my problem is that with the current method signature, VB.NET doesn't allow me to call the method like myObject.Objects[index] – Nation