[release/7.0] Fix ConnectAsync with buffer on Windows (#79866)

* fix ConnectAsync with buffer on Windows

* better tests

Co-authored-by: antonfirsov <antonfir@gmail.com>

Author: github-actions[bot]

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

@@ -307,7 +307,7 @@ internal unsafe SocketError DoOperationConnectEx(Socket socket, SafeSocketHandle
                         handle,
                         PtrSocketAddressBuffer,
                         _socketAddress!.Size,
-                        (IntPtr)((byte*)_singleBufferHandle.Pointer + _offset),
+                        (IntPtr)(bufferPtr + _offset),
                         _count,
                         out int bytesTransferred,
                         overlapped);

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

@@ -7,6 +7,7 @@
 using Xunit;
 using Xunit.Abstractions;
 using Xunit.Sdk;
+using System.Linq;
 
 namespace System.Net.Sockets.Tests
 {
@@ -218,6 +219,58 @@ public ConnectTask(ITestOutputHelper output) : base(output) {}
     public sealed class ConnectEap : Connect<SocketHelperEap>
     {
         public ConnectEap(ITestOutputHelper output) : base(output) {}
+
+        [Theory]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        [InlineData(true)]
+        [InlineData(false)]
+        public async Task ConnectAsync_WithData_DataReceived(bool useArrayApi)
+        {
+            using Socket listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            listener.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+            IPEndPoint serverEp = (IPEndPoint)listener.LocalEndPoint!;
+            listener.Listen();
+
+            var serverTask = Task.Run(async () =>
+            {
+                using Socket handler = await listener.AcceptAsync();
+                using var cts = new CancellationTokenSource(TestSettings.PassingTestTimeout);
+                byte[] recvBuffer = new byte[6];
+                int received = await handler.ReceiveAsync(recvBuffer, SocketFlags.None, cts.Token);
+                Assert.True(received == 4);
+
+                recvBuffer.AsSpan(0, 4).SequenceEqual(new byte[] { 2, 3, 4, 5 });
+            });
+
+            using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+
+            byte[] buffer = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7 };
+            
+            var mre = new ManualResetEventSlim(false);
+            var saea = new SocketAsyncEventArgs();
+            saea.RemoteEndPoint = serverEp;
+
+            // Slice the buffer to test the offset management:
+            if (useArrayApi)
+            {
+                saea.SetBuffer(buffer, 2, 4);
+            }
+            else
+            {
+                saea.SetBuffer(buffer.AsMemory(2, 4));
+            }
+            
+            saea.Completed += (_, __) => mre.Set();
+
+            if (client.ConnectAsync(saea))
+            {
+                Assert.True(mre.Wait(TestSettings.PassingTestTimeout), "Timed out while waiting for connection");
+            }
+
+            Assert.Equal(SocketError.Success, saea.SocketError);
+
+            await serverTask;
+        }
     }
 
     public sealed class ConnectCancellableTask : Connect<SocketHelperCancellableTask>