UWP app System.Net.Sockets - localhost

Question

Hi,

When reading about sockets in UWP, I have seen that we can't use sockets to communicate between 2 apps in the same device with localhost. (Not sure exactly when it works and not).

I just need to clarify on that. If I connect to some other app running in the same device as my UWP app, should I be able to connect to that other app using System.Net.Sockets? Or it is not allowed?
My UWP app will be the client and there will be another app (most probably .net app) which works as the server. So if I call the server from my app like this it seems to work. But based on some articles I am not sure whether there are any restrictions on this. Will this work in a UWP app submitted to Store or will there be issues?

           clientSocket = new Socket(AddressFamily.InterNetwork,  
           SocketType.Dgram, ProtocolType.Udp);  

            //IP address of the server machine  
            IPAddress ipAddress = IPAddress.Parse("127.0.0.1");  
            //Server is listening on port 1000  
            IPEndPoint ipEndPoint = new IPEndPoint(ipAddress, 1000);  

            epServer = (EndPoint)ipEndPoint;  
            clientSocket.BeginSendTo(byteData, 0, byteData.Length, SocketFlags.None, epServer, new AsyncCallback(OnSend), null);

Update : please see the Note here. https://learn.microsoft.com/en-us/windows/uwp/networking/sockets
It says 'Windows disallows establishing a socket connection (Sockets or WinSock) between two UWP apps running on the same machine'. So this applies only for 2 UWP apps? If one is a UWP app and the other is a .net app then it will not be an issue?

Thanks
Madhu

Answer

From a UWP app, you cannot connect to "any" other process on the same system. This is a security restriction, enforced by the firewall.

It may appear to work when you are using Visual Studio and debugging your program, but that's because the firewall rules are relaxed while debugging.

Why is "any" in quote marks? Because apparently you ARE allowed to have a socket between two apps in the same package -- but when you do that, note that you may have trouble shipping the resulting app package in the store.

Answer

Hi,
there's no problem to use sockets in UWP. Try following demo:

XAML

Code

Imports System.Net  
Imports System.Net.Sockets  
Imports System.Threading  
Imports Newtonsoft.Json  
Imports Windows.Networking  
Imports Windows.Networking.Sockets  
  
'''   
''' An empty page that can be used on its own or navigated to within a Frame.  
'''   
Public NotInheritable Class Page13  
  Inherits Page  
  
End Class  
  
Public Class Page13VM  
  Implements INotifyPropertyChanged, IDisposable  
  
  Private _server As New Page13Server  
  
  Private nr As Integer = 0  
  Private hostName As String = "127.0.0.1"  
  Private serverPort As String = "5000"  
  Public Property Display As String = ""  
  Public ReadOnly Property Cmd As ICommand = New RelayCommand(AddressOf CmdExec)  
  Private Async Sub CmdExec(obj As Object)  
    Try  
      Dim socket As New StreamSocket()  
      Dim serverHost As New HostName(hostName)  
      Await socket.ConnectAsync(serverHost, serverPort)  
  
      ' Data object for sending  
      nr += 1  
      Dim d1 As New Page13Data With {.ID = nr, .Info1 = $"Row {nr}", .Info2 = "for response"}  
  
      ' Write data to the server.  
      Using outp As Stream = socket.OutputStream.AsStreamForWrite()  
        Using wrt As New StreamWriter(outp)  
          Await wrt.WriteLineAsync(JsonConvert.SerializeObject(d1))  
          Await wrt.FlushAsync()  
        End Using  
      End Using  
  
      ' Data object für Empfang  
      Dim d2 As Page13Data = Nothing  
  
      ' Read data from erver.  
      Using inp As Stream = socket.InputStream.AsStreamForRead()  
        Using rdr As New StreamReader(inp)  
          Dim resp As String = Await rdr.ReadLineAsync()  
          'Display = resp  
          d2 = JsonConvert.DeserializeObject(Of Page13Data)(resp)  
          Display += $"{Environment.NewLine}Received Object from Server ID:{ d2.ID}, Info1: { d2.Info1}, Info2: { d2.Info2}"  
        End Using  
      End Using  
    Catch ex As Exception  
      Display = ex.ToString()  
    End Try  
    OnPropertyChanged(NameOf(Display))  
  End Sub  
  
  Private Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged  
  Public Sub OnPropertyChanged(Optional propertyName As String = "")  
    RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))  
  End Sub  
  
#Region "IDisposable Support"  
  Private disposedValue As Boolean ' To detect redundant calls  
  
  ' IDisposable  
  Protected Overridable Sub Dispose(disposing As Boolean)  
    Me._server?.Cancel()  
    disposedValue = True  
  End Sub  
  
  Protected Overrides Sub Finalize()  
    Dispose(False)  
    MyBase.Finalize()  
  End Sub  
  
  Public Sub Dispose() Implements IDisposable.Dispose  
    Dispose(True)  
    GC.SuppressFinalize(Me)  
  End Sub  
#End Region  
  
End Class  
  
Public Class Page13Data  
  Public Property ID As Integer  
  Public Property Info1 As String  
  Public Property Info2 As String  
End Class  
  
Friend Class Page13Server  
  Private _listeningPort As Integer = 5000  
  Private _listenningIPAddress As String = "127.0.0.1"  
  Dim source As New CancellationTokenSource()  
  Dim token As CancellationToken  
  
  Public Sub New()  
    token = source.Token  
    Call (New TaskFactory(token)).StartNew(AddressOf Start)  
  End Sub  
  
  Friend Sub Cancel()  
    source.Cancel()  
  End Sub  
  
  Public Async Sub Start()  
    Dim ipAddre As IPAddress = IPAddress.Parse(_listenningIPAddress)  
    Dim listener As TcpListener = New TcpListener(ipAddre, _listeningPort)  
    Try  
      listener.Start()  
      LogMessage("Server is running")  
      LogMessage("Listening on port {_listeningPort}")  
      While True  
        LogMessage("Waiting for connections...")  
        Dim client = Await listener.AcceptTcpClientAsync()  
        HandleConnectionAsync(client)  
      End While  
    Catch ex As Exception  
      LogMessage(ex.ToString())  
    End Try  
  End Sub  
  
  Private Async Sub HandleConnectionAsync(client As TcpClient)  
    client.LingerState.Enabled = False  
    Dim clientInfo As String = client.Client.RemoteEndPoint.ToString()  
    LogMessage(String.Format("Got connection request from {0}", clientInfo))  
    Try  
      Dim d As Page13Data = Nothing  
      Using str As NetworkStream = client.GetStream()  
        Using wrt As New StreamWriter(str)  
          str.ReadTimeout = 600  
          Using rdr As New StreamReader(str)  
            Dim req As String = Await rdr.ReadLineAsync()  
            LogMessage($"Receive from Client:{req}")  
            d = GetData(Of Page13Data)(req)  
            LogMessage($"Received Object from Client ID:{d.ID}, Info1:{d.Info1}, Info2:{d.Info2}")  
            d.Info2 = $"Received from Server for ID {d.ID}"  
            Dim resp As String = GetJSon(Of Page13Data)(d)  
            Await wrt.WriteLineAsync(resp)  
            Await wrt.FlushAsync()  
          End Using  
        End Using  
      End Using  
    Catch exp As Exception  
      LogMessage(exp.ToString())  
    Finally  
      LogMessage($"Closing the client connection - {clientInfo}")  
      client.Close()  
    End Try  
  End Sub  
  Friend Sub LogMessage(msg As String)  
    Debug.WriteLine($"Server:{ msg}")  
  End Sub  
  
  Private Function GetData(Of T)(json As String) As T  
    Return JsonConvert.DeserializeObject(Of T)(json)  
  End Function  
  Private Function GetJSon(Of T)(obj As T) As String  
    Return JsonConvert.SerializeObject(obj)  
  End Function  
  
End Class

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UWP app System.Net.Sockets - localhost

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