Jan Tielens' Bloggings

In one of my previous posts I tried generics both in VB.NET and C#, it turned out both languages support generics the same way. Ofcourse there is the syntax difference between them, and at this time C# has Intellisense that supports generics better, but I'm pretty sure the VB.NET team will catch up. Tonight I explored generics a little bit more; it's possible to add constraints to the generic type. By doing so, you are sure only instances can be created for the generic type, that support for example an interface you want it to. Another possibility is a constraint so the generic types must inherit from a specific base type. Let's say you have a base Entity class, from which your business entity classes inherit from, and a Customer entity class:
Public Class Entity
    Private _id As Long

    Public Property ID() As Long
        Get
            Return _id
        End Get
        Set(ByVal Value As Long)
            _id = Value
        End Set
    End Property
End Class

Public Class Customer
    Inherits Entity
    Private _name As String

    Public Property Name() As String
        Get
            Return _name
        End Get
        Set(ByVal Value As String)
            _name = Value
        End Set
    End Property
End Class

The generic collection class could look like this:
Public Class MyCollection(Of itemType As Entity)
    Inherits CollectionBase

    Default Public Property Item(ByVal index As Integer) As itemType
        Get
            Return CType(MyBase.InnerList(index), itemtype)
        End Get
        Set(ByVal Value As itemType)
            MyBase.InnerList(index) = Value
        End Set
    End Property

    Public Function Add(ByVal item As itemType) As Integer
        Return MyBase.InnerList.Add(item)
    End Function
End Class

Using the generic MyCollection class goes like this:
Dim c As New MyCollection(Of Customer)

So far pretty simple! The fun begins when you want to add multiple constraints. I searched the internet for some examples, but I couldn't find an example (I searched only a minute or so, yes I'm lazy). Luckily Yves was online and he was following a session about generics at U2U, so I asked him if this was possible. His answer was positive (cool!), but he saw this only in C#. So strengthened by the knowledge that this was possible in C#, I searched for the syntax in VB.NET. I tried a few possibilities, and it turned out that you have to use the & character to add more than one constraint. Extending the sample from above with an additional constraint, for example the generic type needs to implement the IComparable interface:
Public Class MyCollection(Of itemType As Entity & IComparable)
...
End Class

By the way, the C# syntax is:
public class List<ItemType> where ItemType : IComparable<ItemType>

Since I couldn't find any official documentation, I cannot garantuee that the syntax showed in this post is correct. So if anyone of MS reads this (by accident probably :-), it would be great if you can confirm this or give the correct syntax.