use dual mode sockets in cases when address family is not set explicitly (dotnet#30036)

* use dual mode sockets in cases when address family is not set explicitly

* update test so it does not fail on platforms without DualMode. In that case it will simply verify that TcpClient without explicit AF can be created without exception.

Author: wfurt

src/System.Net.Sockets/src/System/Net/Sockets/TCPClient.cs

@@ -20,7 +20,7 @@ public class TcpClient : IDisposable
         private bool _active;
 
         // Initializes a new instance of the System.Net.Sockets.TcpClient class.
-        public TcpClient() : this(AddressFamily.InterNetwork)
+        public TcpClient() : this(AddressFamily.Unknown)
         {
         }
 
@@ -31,7 +31,8 @@ public TcpClient(AddressFamily family)
 
             // Validate parameter
             if (family != AddressFamily.InterNetwork &&
-                family != AddressFamily.InterNetworkV6)
+                family != AddressFamily.InterNetworkV6 &&
+                family != AddressFamily.Unknown)
             {
                 throw new ArgumentException(SR.Format(SR.net_protocol_invalid_family, "TCP"), nameof(family));
             }
@@ -111,7 +112,11 @@ protected bool Active
         public Socket Client
         {
             get { return _clientSocket; }
-            set { _clientSocket = value; }
+            set
+            {
+                _clientSocket = value;
+                _family = _clientSocket?.AddressFamily ?? AddressFamily.Unknown;
+            }
         }
 
         public bool Connected => _clientSocket?.Connected ?? false;
@@ -186,7 +191,7 @@ public void Connect(string hostname, int port)
                             _active = true;
                             break;
                         }
-                        else if (address.AddressFamily == _family)
+                        else if (address.AddressFamily == _family || _family == AddressFamily.Unknown)
                         {
                             // Only use addresses with a matching family
                             Connect(new IPEndPoint(address, port));
@@ -257,6 +262,7 @@ public void Connect(IPEndPoint remoteEP)
             }
 
             Client.Connect(remoteEP);
+            _family = Client.AddressFamily;
             _active = true;
 
             if (NetEventSource.IsEnabled) NetEventSource.Exit(this);
@@ -267,6 +273,7 @@ public void Connect(IPAddress[] ipAddresses, int port)
             if (NetEventSource.IsEnabled) NetEventSource.Enter(this, ipAddresses);
 
             Client.Connect(ipAddresses, port);
+            _family = Client.AddressFamily;
             _active = true;
 
             if (NetEventSource.IsEnabled) NetEventSource.Exit(this);
@@ -468,7 +475,19 @@ public bool NoDelay
         private void InitializeClientSocket()
         {
             Debug.Assert(_clientSocket == null);
-            _clientSocket = new Socket(_family, SocketType.Stream, ProtocolType.Tcp);
+            if (_family == AddressFamily.Unknown)
+            {
+                // If AF was not explicitly set try to initialize dual mode socket or fall-back to IPv4.
+                _clientSocket = new Socket(SocketType.Stream, ProtocolType.Tcp);
+                if (_clientSocket.AddressFamily == AddressFamily.InterNetwork)
+                {
+                    _family = AddressFamily.InterNetwork;
+                }
+            }
+            else
+            {
+                _clientSocket = new Socket(_family, SocketType.Stream, ProtocolType.Tcp);
+            }
         }
     }
 }

src/System.Net.Sockets/tests/FunctionalTests/TcpClientTest.cs

@@ -25,7 +25,6 @@ public TcpClientTest(ITestOutputHelper output)
         [InlineData(AddressFamily.DataLink)]
         [InlineData(AddressFamily.NetBios)]
         [InlineData(AddressFamily.Unix)]
-        [InlineData(AddressFamily.Unknown)]
         public void Ctor_InvalidFamily_Throws(AddressFamily family)
         {
             AssertExtensions.Throws<ArgumentException>("family", () => new TcpClient(family));
@@ -430,6 +429,33 @@ public async Task Dispose_CancelsConnectAsync(bool connectByName)
             }
         }
 
+        [Fact]
+        public void Connect_Dual_Success()
+        {
+            if (!Socket.OSSupportsIPv6)
+            {
+                return;
+            }
+
+            using (var server = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp))
+            {
+                // Set up a server socket to which to connect
+                server.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
+                server.Listen(1);
+                var endpoint = (IPEndPoint)server.LocalEndPoint;
+
+                using (TcpClient client = new TcpClient())
+                {
+                    // Some platforms may not support IPv6 dual mode and they should fall-back to IPv4
+                    // without throwing exception. However in such case attempt to connect to IPv6 would still fail.
+                    if (client.Client.AddressFamily == AddressFamily.InterNetworkV6 && client.Client.DualMode)
+                    {
+                        client.Connect(endpoint);
+                    }
+                }
+            }
+        }
+
         private sealed class DerivedTcpClient : TcpClient
         {
             public new bool Active