Question

Okay, i’ve been trying to toy around with generics in vb.net in an example project. At the moment, it looks like this:

I have an interface called IRow (and a class that implements it as a Datarow).

A second interface is ICanGetByRow, which looks like this:

Public Interface ICanGetByRow(Of T)

    Function GetByRow(ByVal Row As IRow) As T

End Interface

The function simply takes an IRow and converts it to T. Thats easy enough. Now, for easier access, i want to implement a function in the IRow interface which takes the row and converts it into said ICanGetByRow.

My interface was enhanced by the following function

Function GetObj(Of T As ICanGetByRow(Of T))() As T

You can probably see the problem. If i implemented it like this:

Public Function GetObj(Of T As ICanGetByRow(Of T))() As T Implements IRow.GetObj
    Dim foo As New T
    foo.GetByRow(Me)
    Return foo
End Function

i wouldn’t be allowed to construct a new T, and when i tried to make it work by telling the generic function that my interface has a constructor, he wouldn’t let me invoke GetByRow anymore.

Public Function GetObj(Of T As New, ICanGetByRow)() As T Implements IRow.GetObj
    Dim foo As New T
    foo.GetByRow(Me)
    Return foo
End Function

I think my problem is that i have no idea how to tell a generic function that it will get a ICanGetByRow(of T) which has a constructor, and i’m probably doing it wrong.

Answer 1

The syntax is a bit, well, odd. You have to list multiple constraints with { braces }. Like this:

Interface ICanGetByRow(Of T)
    Sub GetByRow(ByVal arg As IRow)
End Interface
Interface IRow
    Function GetObj(Of T As {New, ICanGetByRow(Of T)})() As T
End Interface

Class Impl
    Implements IRow
    Public Function GetObj(Of T As {New, ICanGetByRow(Of T)})() As T Implements IRow.GetObj
        Dim foo As New T
        foo.GetByRow(Me)
        Return foo
    End Function
End Class