>Compiles OK for me with VS2012 targeting 4.5. What version of VS and .NET are you using?

Thanks, this works.

Would you know why the first one does not require this syntax while the second one does?

    Public Overrides Function CanBuildChannelFactory(Of TChannel)(context As System.ServiceModel.Channels.BindingContext) As Boolean
        If context Is Nothing Then
            Throw New ArgumentNullException("context")
        End If

        Return context.CanBuildInnerChannelFactory(Of TChannel)()
    End Function

    Public Overrides Function BuildChannelListener(Of TChannel As_
     {Class, System.ServiceModel.Channels.IChannel})(context As System.ServiceModel.Channels.BindingContext) As _
     System.ServiceModel.Channels.IChannelListener(Of TChannel)
        If context Is Nothing Then
            Throw New ArgumentNullException("context")
        End If

        context.BindingParameters.Add(Me)
        Return context.BuildInnerChannelListener(Of TChannel)()
    End Function