fix SendTo with SocketAsyncEventArgs

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

@@ -677,7 +677,6 @@ public ValueTask<int> SendToAsync(ReadOnlyMemory<byte> buffer, SocketFlags socke
             Debug.Assert(saea.BufferList == null);
             saea.SetBuffer(MemoryMarshal.AsMemory(buffer));
             saea.SocketFlags = socketFlags;
-            saea._socketAddress = null;
             saea.RemoteEndPoint = remoteEP;
             saea.WrapExceptionsForNetworkStream = false;
             return saea.SendToAsync(this, cancellationToken);
@@ -709,8 +708,17 @@ public ValueTask<int> SendToAsync(ReadOnlyMemory<byte> buffer, SocketFlags socke
             saea.SetBuffer(MemoryMarshal.AsMemory(buffer));
             saea.SocketFlags = socketFlags;
             saea._socketAddress = socketAddress;
+            saea.RemoteEndPoint = null;
             saea.WrapExceptionsForNetworkStream = false;
-            return saea.SendToAsync(this, cancellationToken);
+            try
+            {
+                return saea.SendToAsync(this, cancellationToken);
+            }
+            finally
+            {
+                // detach user provided SA so we do not accidentally stomp on it later.
+                saea._socketAddress = null;
+            }
         }
 
         /// <summary>

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

@@ -3095,14 +3095,22 @@ private bool SendToAsync(SocketAsyncEventArgs e, CancellationToken cancellationT
             ArgumentNullException.ThrowIfNull(e);
 
             EndPoint? endPointSnapshot = e.RemoteEndPoint;
-            if (e._socketAddress == null)
+
+            // RemoteEndPoint should be set unless somebody used SendTo with their own SA.
+            // In that case RemoteEndPoint will be null and we take provided SA as given.
+            if (endPointSnapshot == null && e._socketAddress == null)
             {
-                if (endPointSnapshot == null)
-                {
-                    throw new ArgumentException(SR.Format(SR.InvalidNullArgument, "e.RemoteEndPoint"), nameof(e));
-                }
+                throw new ArgumentException(SR.Format(SR.InvalidNullArgument, "e.RemoteEndPoint"), nameof(e));
+            }
 
-                // Prepare SocketAddress
+            if (e._socketAddress != null && endPointSnapshot is IPEndPoint ipep && e._socketAddress.Family == endPointSnapshot?.AddressFamily)
+            {
+                // we have matching SocketAddress. Since this is only used internally, it is ok to overwrite it without
+                ipep.Serialize(e._socketAddress.Buffer.Span);
+            }
+            else if (endPointSnapshot != null)
+            {
+                // Prepare new SocketAddress
                 e._socketAddress = Serialize(ref endPointSnapshot);
             }
 

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

@@ -923,7 +923,12 @@ internal void FinishOperationSyncSuccess(int bytesTransferred, SocketFlags flags
                 case SocketAsyncOperation.ReceiveFrom:
                     // Deal with incoming address.
                     UpdateReceivedSocketAddress(_socketAddress!);
-                    if (_remoteEndPoint != null && !SocketAddressExtensions.Equals(_socketAddress!, _remoteEndPoint))
+                    if (_remoteEndPoint == null)
+                    {
+                        // detach user provided SA as it was updated in place.
+                        _socketAddress = null;
+                    }
+                    else if (!SocketAddressExtensions.Equals(_socketAddress!, _remoteEndPoint))
                     {
                         try
                         {

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

@@ -173,6 +173,35 @@ public void SendToAsync_NullAsyncEventArgs_Throws_ArgumentNullException()
     public sealed class SendTo_Task : SendTo<SocketHelperTask>
     {
         public SendTo_Task(ITestOutputHelper output) : base(output) { }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task SendTo_DifferentEP_Success(bool ipv4)
+        {
+            IPAddress address = ipv4 ? IPAddress.Loopback : IPAddress.IPv6Loopback;
+            IPEndPoint remoteEp = new IPEndPoint(address, 0);
+
+            using Socket receiver1 = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            using Socket receiver2 = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            using Socket sender = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+
+            receiver1.BindToAnonymousPort(address);
+            receiver2.BindToAnonymousPort(address);
+
+            byte[] sendBuffer = new byte[32];
+            var receiveInternalBuffer = new byte[sendBuffer.Length];
+            ArraySegment<byte> receiveBuffer = new ArraySegment<byte>(receiveInternalBuffer, 0, receiveInternalBuffer.Length);
+
+
+            await sender.SendToAsync(sendBuffer, SocketFlags.None, receiver1.LocalEndPoint);
+            SocketReceiveFromResult result = await ReceiveFromAsync(receiver1, receiveBuffer, remoteEp).WaitAsync(TestSettings.PassingTestTimeout);
+            Assert.Equal(sendBuffer.Length, result.ReceivedBytes);
+
+            await sender.SendToAsync(sendBuffer, SocketFlags.None, receiver2.LocalEndPoint);
+            result = await ReceiveFromAsync(receiver2, receiveBuffer, remoteEp).WaitAsync(TestSettings.PassingTestTimeout);
+            Assert.Equal(sendBuffer.Length, result.ReceivedBytes);
+        }
     }
 
     public sealed class SendTo_CancellableTask : SendTo<SocketHelperCancellableTask>

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

@@ -895,5 +895,52 @@ void CreateSocketAsyncEventArgs() // separated out so that JIT doesn't extend li
                 return cwt.Count() == 0; // validate that the cwt becomes empty
             }, 30_000));
         }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task SendTo_DifferentEP_Success(bool ipv4)
+        {
+            IPAddress address = ipv4 ? IPAddress.Loopback : IPAddress.IPv6Loopback;
+            IPEndPoint remoteEp = new IPEndPoint(address, 0);
+
+            using Socket receiver1 = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            using Socket receiver2 = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            using Socket sender = new Socket(address.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+
+            receiver1.BindToAnonymousPort(address);
+            receiver2.BindToAnonymousPort(address);
+
+            byte[] sendBuffer = new byte[32];
+            var receiveInternalBuffer = new byte[sendBuffer.Length];
+            ArraySegment<byte> receiveBuffer = new ArraySegment<byte>(receiveInternalBuffer, 0, receiveInternalBuffer.Length);
+
+            using SocketAsyncEventArgs saea = new SocketAsyncEventArgs();
+            ManualResetEventSlim mres = new ManualResetEventSlim(false); 
+
+            saea.SetBuffer(sendBuffer);
+            saea.RemoteEndPoint = receiver1.LocalEndPoint;
+            saea.Completed += delegate { mres.Set(); };
+            if (sender.SendToAsync(saea))
+            {
+                // did not finish synchronously.
+                mres.Wait();
+            }
+
+            SocketReceiveFromResult result = await receiver1.ReceiveFromAsync(receiveBuffer, remoteEp).WaitAsync(TestSettings.PassingTestTimeout);
+            Assert.Equal(sendBuffer.Length, result.ReceivedBytes);
+            mres.Reset();
+
+
+            saea.RemoteEndPoint = receiver2.LocalEndPoint;
+            if (sender.SendToAsync(saea))
+            {
+                // did not finish synchronously.
+                mres.Wait();
+            }
+
+            result = await receiver2.ReceiveFromAsync(receiveBuffer, remoteEp).WaitAsync(TestSettings.PassingTestTimeout);
+            Assert.Equal(sendBuffer.Length, result.ReceivedBytes);
+        }
     }
 }