>>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
>

Because of how the base generic types are defined:

Public Overridable Function CanBuildChannelFactory(Of TChannel) ( _
	context As BindingContext _
) As Boolean

Public Overridable Function BuildChannelListener(Of TChannel As {Class, IChannel}) ( _
	listenUriBaseAddress As Uri, _
	ParamArray parameters As Object() _
) As IChannelListener(Of TChannel)

http://msdn.microsoft.com/en-us/library/ms405436(v=vs.110).aspx
http://msdn.microsoft.com/en-us/library/ms405443(v=vs.110).aspx