<CallbackBehavior(UseSynchronizationContext:=False)> _
Public Class Service
    Implements ICallback

    Public Server As IServer

    Sub New()

    End Sub

    Public Sub OpenConnect()
        Dim NTB As New NetTcpBinding()
        NTB.Security.Mode = SecurityMode.Transport
        NTB.Security.Transport.ProtectionLevel = Net.Security.ProtectionLevel.None
        NTB.Security.Transport.ClientCredentialType = TcpClientCredentialType.Windows
        NTB.Security.Message.ClientCredentialType = TcpClientCredentialType.Windows
        Dim DCF As DuplexChannelFactory(Of IServer) = New DuplexChannelFactory(Of IServer)(New InstanceContext(Me), NTB, "net.tcp://192.168.1.2:8080/wcf_server")
        Server = DCF.CreateChannel(New EndpointAddress("net.tcp://192.168.1.2:8080/wcf_server"))
    End Sub

    Sub Ping() Implements ICallback.Ping
    End Sub

    Sub ClientExit() Implements ICallback.ClientExit
        Application.Exit()
    End Sub

    Sub GoMessage(ByVal Message As Mess) Implements ICallback.GoMessage

    End Sub
End Class

    Sub GoMessage(ByVal Message As Mess) Implements ICallback.GoMessage
        RaiseEvent MessageEnter(Me, New GoMessageEventArgs(Message))
    End Sub

    Public Event MessageEnter As EventHandler(Of GoMessageEventArgs)
End Class

Public Class GoMessageEventArgs
    Inherits EventArgs

    Private pr_Message As Mess

    Sub New(ByVal p_Message As Mess)
        pr_Message = p_Message
    End Sub

    Public Property Message() As Mess
        Get
            Return pr_Message
        End Get
        Set(ByVal value As Mess)
            pr_Message = value
        End Set
    End Property
End Class

'в каком-нибудь классе (форме)

'так подписываемся
    Private Sub F_MAIN_MENU_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
AddHandler S.MessageEnter, AddressOf GoMessage
    End Sub

    Private Sub GoMessage(ByVal sender As System.Object, ByVal e As GoMessageEventArgs)
        MsgBox(e.Message.TextMessage)
    End Sub