Auto-enable SO_REUSE_UNICASTPORT for ConnectEx (#54903)

Fixes #48219.

Author: antonfirsov

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

@@ -235,9 +235,34 @@ partial void WildcardBindForConnectIfNecessary(AddressFamily addressFamily)
 
             if (NetEventSource.Log.IsEnabled()) NetEventSource.Info(this, csep.IPEndPoint);
 
+            if (_socketType == SocketType.Stream && _protocolType == ProtocolType.Tcp)
+            {
+                EnableReuseUnicastPort();
+            }
+
             DoBind(csep.IPEndPoint, csep.SocketAddress);
         }
 
+        private void EnableReuseUnicastPort()
+        {
+            // By enabling SO_REUSE_UNICASTPORT, we defer actual port allocation until the ConnectEx call,
+            // so it can bind to ports from the Windows auto-reuse port range, if configured by an admin.
+            // The socket option is supported on Windows 10+, we are ignoring the SocketError in case setsockopt fails.
+            int optionValue = 1;
+            SocketError error = Interop.Winsock.setsockopt(
+                _handle,
+                SocketOptionLevel.Socket,
+                SocketOptionName.ReuseUnicastPort,
+                ref optionValue,
+                sizeof(int));
+
+            if (NetEventSource.Log.IsEnabled() && error != SocketError.Success)
+            {
+                error = SocketPal.GetLastSocketError();
+                NetEventSource.Info($"Enabling SO_REUSE_UNICASTPORT failed with error code: {error}");
+            }
+        }
+
         internal unsafe bool ConnectEx(SafeSocketHandle socketHandle,
             IntPtr socketAddress,
             int socketAddressSize,