Socket.Windows: support ConnectAsync(SocketAsyncEventArgs) for UDP, and Unix sockets

src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs

@@ -2090,11 +2090,17 @@ public IAsyncResult BeginConnect(EndPoint remoteEP, AsyncCallback? callback, obj
 
         private bool CanUseConnectEx(EndPoint remoteEP)
         {
-            return (_socketType == SocketType.Stream) &&
-                (_rightEndPoint != null || remoteEP.GetType() == typeof(IPEndPoint));
-        }
+            Debug.Assert(remoteEP.GetType() != typeof(DnsEndPoint));
 
+            // ConnectEx supports connection-oriented sockets.
+            // The socket must be bound before calling ConnectEx.
+            //     In case of IPEndPoint, the Socket will be bound using WildcardBindForConnectIfNecessary.
+            // Unix sockets are not supported by ConnectEx.
 
+            return (_socketType == SocketType.Stream) &&
+                   (_rightEndPoint != null || remoteEP.GetType() == typeof(IPEndPoint)) &&
+                   (remoteEP.AddressFamily != AddressFamily.Unix);
+        }
 
         internal IAsyncResult UnsafeBeginConnect(EndPoint remoteEP, AsyncCallback? callback, object? state, bool flowContext = false)
         {
@@ -3817,7 +3823,15 @@ private bool ConnectAsync(SocketAsyncEventArgs e, bool userSocket)
                 SocketError socketError = SocketError.Success;
                 try
                 {
-                    socketError = e.DoOperationConnect(this, _handle);
+                    if (CanUseConnectEx(endPointSnapshot))
+                    {
+                        socketError = e.DoOperationConnectEx(this, _handle);
+                    }
+                    else
+                    {
+                        // For connectionless protocols, Connect is not an I/O call.
+                        socketError = e.DoOperationConnect(this, _handle);
+                    }
                 }
                 catch
                 {

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.Unix.cs

@@ -38,20 +38,6 @@ private void SetupMultipleBuffers()
 
         private void CompleteCore() { }
 
-        private void FinishOperationSync(SocketError socketError, int bytesTransferred, SocketFlags flags)
-        {
-            Debug.Assert(socketError != SocketError.IOPending);
-
-            if (socketError == SocketError.Success)
-            {
-                FinishOperationSyncSuccess(bytesTransferred, flags);
-            }
-            else
-            {
-                FinishOperationSyncFailure(socketError, bytesTransferred, flags);
-            }
-        }
-
         private void AcceptCompletionCallback(IntPtr acceptedFileDescriptor, byte[] socketAddress, int socketAddressSize, SocketError socketError)
         {
             CompleteAcceptOperation(acceptedFileDescriptor, socketAddress, socketAddressSize, socketError);
@@ -95,6 +81,9 @@ private void ConnectCompletionCallback(SocketError socketError)
             CompletionCallback(0, SocketFlags.None, socketError);
         }
 
+        internal unsafe SocketError DoOperationConnectEx(Socket socket, SafeSocketHandle handle)
+            => DoOperationConnect(socket, handle);
+
         internal unsafe SocketError DoOperationConnect(Socket socket, SafeSocketHandle handle)
         {
             SocketError socketError = handle.AsyncContext.ConnectAsync(_socketAddress!.Buffer, _socketAddress.Size, ConnectCompletionCallback);

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.Windows.cs

@@ -272,6 +272,14 @@ internal unsafe SocketError DoOperationAccept(Socket socket, SafeSocketHandle ha
         }
 
         internal unsafe SocketError DoOperationConnect(Socket socket, SafeSocketHandle handle)
+        {
+            // Called for connectionless protocols.
+            SocketError socketError = SocketPal.Connect(handle, _socketAddress!.Buffer, _socketAddress.Size);
+            FinishOperationSync(socketError, 0, SocketFlags.None);
+            return socketError;
+        }
+
+        internal unsafe SocketError DoOperationConnectEx(Socket socket, SafeSocketHandle handle)
         {
             // ConnectEx uses a sockaddr buffer containing the remote address to which to connect.
             // It can also optionally take a single buffer of data to send after the connection is complete.
@@ -1160,6 +1168,13 @@ private unsafe SocketError FinishOperationConnect()
         {
             try
             {
+                if (_currentSocket!.SocketType != SocketType.Stream)
+                {
+                    // With connectionless sockets, regular connect is used instead of ConnectEx,
+                    // attempting to set SO_UPDATE_CONNECT_CONTEXT will result in an error.
+                    return SocketError.Success;
+                }
+
                 // Update the socket context.
                 SocketError socketError = Interop.Winsock.setsockopt(
                     _currentSocket!.SafeHandle,

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.cs

@@ -780,5 +780,19 @@ internal void FinishOperationAsyncSuccess(int bytesTransferred, SocketFlags flag
                 ExecutionContext.Run(context, s_executionCallback, this);
             }
         }
+
+        private void FinishOperationSync(SocketError socketError, int bytesTransferred, SocketFlags flags)
+        {
+            Debug.Assert(socketError != SocketError.IOPending);
+
+            if (socketError == SocketError.Success)
+            {
+                FinishOperationSyncSuccess(bytesTransferred, flags);
+            }
+            else
+            {
+                FinishOperationSyncFailure(socketError, bytesTransferred, flags);
+            }
+        }
     }
 }

src/libraries/System.Net.Sockets/tests/FunctionalTests/Connect.cs

@@ -30,6 +30,40 @@ public async Task Connect_Success(IPAddress listenAt)
             }
         }
 
+        [Theory]
+        [MemberData(nameof(Loopbacks))]
+        public async Task Connect_Udp_Success(IPAddress listenAt)
+        {
+            using Socket listener = new Socket(listenAt.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            using Socket client = new Socket(listenAt.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            listener.Bind(new IPEndPoint(listenAt, 0));
+
+            await ConnectAsync(client, new IPEndPoint(listenAt, ((IPEndPoint)listener.LocalEndPoint).Port));
+            Assert.True(client.Connected);
+        }
+
+        [Theory]
+        [MemberData(nameof(Loopbacks))]
+        public async Task Connect_Dns_Success(IPAddress listenAt)
+        {
+            // On some systems (like Ubuntu 16.04 and Ubuntu 18.04) "localhost" doesn't resolve to '::1'.
+            if (Array.IndexOf(Dns.GetHostAddresses("localhost"), listenAt) == -1)
+            {
+                return;
+            }
+
+            int port;
+            using (SocketTestServer.SocketTestServerFactory(SocketImplementationType.Async, listenAt, out port))
+            {
+                using (Socket client = new Socket(listenAt.AddressFamily, SocketType.Stream, ProtocolType.Tcp))
+                {
+                    Task connectTask = ConnectAsync(client, new DnsEndPoint("localhost", port));
+                    await connectTask;
+                    Assert.True(client.Connected);
+                }
+            }
+        }
+
         [OuterLoop]
         [Theory]
         [MemberData(nameof(Loopbacks))]

src/libraries/System.Net.Sockets/tests/FunctionalTests/UnixDomainSocketTest.cs

@@ -21,7 +21,6 @@ public void OSSupportsUnixDomainSockets_ReturnsCorrectValue()
             Assert.Equal(PlatformSupportsUnixDomainSockets, Socket.OSSupportsUnixDomainSockets);
         }
 
-        [PlatformSpecific(~TestPlatforms.Windows)] // Windows doesn't currently support ConnectEx with domain sockets
         [ConditionalFact(nameof(PlatformSupportsUnixDomainSockets))]
         public async Task Socket_ConnectAsyncUnixDomainSocketEndPoint_Success()
         {
@@ -100,7 +99,7 @@ public async Task Socket_ConnectAsyncUnixDomainSocketEndPoint_NotServer()
                     }
 
                     Assert.Equal(
-                        RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? SocketError.InvalidArgument : SocketError.AddressNotAvailable,
+                        RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? SocketError.ConnectionRefused : SocketError.AddressNotAvailable,
                         args.SocketError);
                 }
             }