Use `NetworkStream` from CoreFX. (#16889)

* Use `NetworkStream` from CoreFX.

This brings the `NetworkStream` class and related tests from CoreFX.

* `SocketAsyncEventArgs`: partially replace the following properties
  with the CoreFX version: `Buffer`, `BufferList`, `Offset`, `SetBuffer ()`
  as well as the internal `CopyBufferFrom ()`.

* `SocketAsyncResult`: change `Buffer` from `byte[]` to `Memory<byte>`.

* Add internal `Socket` overloads:
   - `Receive (Memory<byte> buffer, int, int, SocketFlags, out SocketError)`
   - `ReceiveFrom (Memory<byte>, int, int, SocketFlags, ref EndPoint, out SocketError)`
   - `SendTo (Memory<byte>, int, int, SocketFlags, EndPoint)`

* Implement async task overloads from CoreFX (`Socket.Tasks.cs`) to `Socket`:
   - `int Read(Span<byte>)`
   - `Task<int> ReadAsync(byte[], int, int, CancellationToken)`
   - `ValueTask<int> ReadAsync(Memory<byte>, CancellationToken)`
   - `int ReadByte()`
   - `void Write(ReadOnlySpan<byte>)`
   - `Task WriteAsync(byte[], int, int, CancellationToken)`
   - `ValueTask WriteAsync(ReadOnlyMemory<byte>, CancellationToken)`
   - `void WriteByte(byte)`

* remove internal unused `PooledStream` and `ConnectionPool` APIs.

* bring new tests from CoreFX:
  - `System.Net.Sockets/tests/FunctionalTests/NetworkStreamTest.cs`
  - `System.Net.Sockets/tests/FunctionalTests/NetworkStreamTest.netcoreapp.cs`

* corlib: add `System.IO.FileSystem` and `System.Net.Sockets` resources and update SR.

* Update Socket.cs

* [csproj] Update project files

* Bump API snapshot submodule

11 files changed, 323 insertions, 96 deletions

diff --git a/mcs/class/System/System.Net.Sockets/Socket.cs b/mcs/class/System/System.Net.Sockets/Socket.cs
index b130853ff19..3c369158934 100644
--- a/mcs/class/System/System.Net.Sockets/Socket.cs
+++ b/mcs/class/System/System.Net.Sockets/Socket.cs
@@ -740,7 +740,7 @@ namespace System.Net.Sockets
 
 			sockares.CheckIfThrowDelayedException ();
 
-			buffer = sockares.Buffer;
+			buffer = sockares.Buffer.ToArray ();
 			bytesTransferred = sockares.Total;
 
 			return sockares.AcceptedSocket;
@@ -1408,6 +1408,29 @@ namespace System.Net.Sockets
 			return ret;
 		}
 
+		int Receive (Memory<byte> buffer, int offset, int size, SocketFlags socketFlags, out SocketError errorCode)
+		{
+			ThrowIfDisposedAndClosed ();
+
+			int nativeError;
+			int ret;
+			unsafe {
+				using (var handle = buffer.Slice (offset, size).Pin ()) {
+					ret = Receive_internal (m_Handle, (byte*)handle.Pointer, size, socketFlags, out nativeError, is_blocking);
+				}
+			}
+
+			errorCode = (SocketError) nativeError;
+			if (errorCode != SocketError.Success && errorCode != SocketError.WouldBlock && errorCode != SocketError.InProgress) {
+				is_connected = false;
+				is_bound = false;
+			} else {
+				is_connected = true;
+			}
+
+			return ret;
+		}
+
 		[CLSCompliant (false)]
 		public int Receive (IList<ArraySegment<byte>> buffers, SocketFlags socketFlags, out SocketError errorCode)
 		{
@@ -1486,10 +1509,10 @@ namespace System.Net.Sockets
 			// LAME SPEC: the ArgumentException is never thrown, instead an NRE is
 			// thrown when e.Buffer and e.BufferList are null (works fine when one is
 			// set to a valid object)
-			if (e.Buffer == null && e.BufferList == null)
+			if (e.MemoryBuffer.Equals (default) && e.BufferList == null)
 				throw new NullReferenceException ("Either e.Buffer or e.BufferList must be valid buffers.");
 
-			if (e.Buffer == null) {
+			if (e.BufferList != null) {
 				InitSocketAsyncEventArgs (e, ReceiveAsyncCallback, e, SocketOperation.ReceiveGeneric);
 
 				e.socket_async_result.Buffers = e.BufferList;
@@ -1498,7 +1521,7 @@ namespace System.Net.Sockets
 			} else {
 				InitSocketAsyncEventArgs (e, ReceiveAsyncCallback, e, SocketOperation.Receive);
 
-				e.socket_async_result.Buffer = e.Buffer;
+				e.socket_async_result.Buffer = e.MemoryBuffer;
 				e.socket_async_result.Offset = e.Offset;
 				e.socket_async_result.Size = e.Count;
 
@@ -1554,8 +1577,8 @@ namespace System.Net.Sockets
 
 			try {
 				unsafe {
-					fixed (byte* pbuffer = sockares.Buffer) {
-						total = Receive_internal (sockares.socket.m_Handle, &pbuffer[sockares.Offset], sockares.Size, sockares.SockFlags, out sockares.error, sockares.socket.is_blocking);
+					using (var pbuffer = sockares.Buffer.Slice (sockares.Offset, sockares.Size).Pin ()) {
+						total = Receive_internal (sockares.socket.m_Handle, (byte*)pbuffer.Pointer, sockares.Size, sockares.SockFlags, out sockares.error, sockares.socket.is_blocking);
 					}
 				}
 			} catch (Exception e) {
@@ -1709,6 +1732,44 @@ namespace System.Net.Sockets
 			return cnt;
 		}
 
+		int ReceiveFrom (Memory<byte> buffer, int offset, int size, SocketFlags socketFlags, ref EndPoint remoteEP, out SocketError errorCode)
+		{
+			SocketAddress sockaddr = remoteEP.Serialize();
+
+			int nativeError;
+			int cnt;
+			unsafe {
+				using (var handle = buffer.Slice (offset, size).Pin ()) {
+					cnt = ReceiveFrom_internal (m_Handle, (byte*)handle.Pointer, size, socketFlags, ref sockaddr, out nativeError, is_blocking);
+				}
+			}
+
+			errorCode = (SocketError) nativeError;
+			if (errorCode != SocketError.Success) {
+				if (errorCode != SocketError.WouldBlock && errorCode != SocketError.InProgress) {
+					is_connected = false;
+				} else if (errorCode == SocketError.WouldBlock && is_blocking) { // This might happen when ReceiveTimeout is set
+					errorCode = SocketError.TimedOut;
+				}
+
+				return 0;
+			}
+
+			is_connected = true;
+			is_bound = true;
+
+			/* If sockaddr is null then we're a connection oriented protocol and should ignore the
+			 * remoteEP parameter (see MSDN documentation for Socket.ReceiveFrom(...) ) */
+			if (sockaddr != null) {
+				/* Stupidly, EndPoint.Create() is an instance method */
+				remoteEP = remoteEP.Create (sockaddr);
+			}
+
+			seed_endpoint = remoteEP;
+
+			return cnt;
+		}
+
 		public bool ReceiveFromAsync (SocketAsyncEventArgs e)
 		{
 			ThrowIfDisposedAndClosed ();
@@ -1989,10 +2050,10 @@ namespace System.Net.Sockets
 
 			ThrowIfDisposedAndClosed ();
 
-			if (e.Buffer == null && e.BufferList == null)
+			if (e.MemoryBuffer.Equals (default) && e.BufferList == null)
 				throw new NullReferenceException ("Either e.Buffer or e.BufferList must be valid buffers.");
 
-			if (e.Buffer == null) {
+			if (e.BufferList != null) {
 				InitSocketAsyncEventArgs (e, SendAsyncCallback, e, SocketOperation.SendGeneric);
 
 				e.socket_async_result.Buffers = e.BufferList;
@@ -2001,7 +2062,7 @@ namespace System.Net.Sockets
 			} else {
 				InitSocketAsyncEventArgs (e, SendAsyncCallback, e, SocketOperation.Send);
 
-				e.socket_async_result.Buffer = e.Buffer;
+				e.socket_async_result.Buffer = e.MemoryBuffer;
 				e.socket_async_result.Offset = e.Offset;
 				e.socket_async_result.Size = e.Count;
 
@@ -2059,8 +2120,8 @@ namespace System.Net.Sockets
 
 			try {
 				unsafe {
-					fixed (byte *pbuffer = sockares.Buffer) {
-						total = Socket.Send_internal (sockares.socket.m_Handle, &pbuffer[sockares.Offset], sockares.Size, sockares.SockFlags, out sockares.error, false);
+					using (var pbuffer = sockares.Buffer.Slice (sockares.Offset, sockares.Size).Pin ()) {
+						total = Socket.Send_internal (sockares.socket.m_Handle, (byte*)pbuffer.Pointer, sockares.Size, sockares.SockFlags, out sockares.error, false);
 					}
 				}
 			} catch (Exception e) {
@@ -2211,6 +2272,35 @@ namespace System.Net.Sockets
 			return ret;
 		}
 
+		int SendTo (Memory<byte> buffer, int offset, int size, SocketFlags socketFlags, EndPoint remoteEP)
+		{
+			ThrowIfDisposedAndClosed ();
+
+			if (remoteEP == null)
+				throw new ArgumentNullException("remoteEP");
+
+			int error;
+			int ret;
+			unsafe {
+				using (var pbuffer = buffer.Slice (offset, size).Pin ()) {
+					ret = SendTo_internal (m_Handle, (byte*)pbuffer.Pointer, size, socketFlags, remoteEP.Serialize (), out error, is_blocking);
+				}
+			}
+
+			SocketError err = (SocketError) error;
+			if (err != 0) {
+				if (err != SocketError.WouldBlock && err != SocketError.InProgress)
+					is_connected = false;
+				throw new SocketException (error);
+			}
+
+			is_connected = true;
+			is_bound = true;
+			seed_endpoint = remoteEP;
+
+			return ret;
+		}
+
 		public bool SendToAsync (SocketAsyncEventArgs e)
 		{
 			// NO check is made whether e != null in MS.NET (NRE is thrown in such case)
diff --git a/mcs/class/System/System.Net.Sockets/SocketAsyncEventArgs.cs b/mcs/class/System/System.Net.Sockets/SocketAsyncEventArgs.cs
index def7c46d6f3..69510e35ebf 100644
--- a/mcs/class/System/System.Net.Sockets/SocketAsyncEventArgs.cs
+++ b/mcs/class/System/System.Net.Sockets/SocketAsyncEventArgs.cs
@@ -36,7 +36,7 @@ using System.Threading;
 
 namespace System.Net.Sockets
 {
-	public class SocketAsyncEventArgs : EventArgs, IDisposable
+	public partial class SocketAsyncEventArgs : EventArgs, IDisposable
 	{
 		bool disposed;
 
@@ -56,33 +56,11 @@ namespace System.Net.Sockets
 			set;
 		}
 
-		public byte[] Buffer {
-			get;
-			private set;
-		}
-
-		public Memory<byte> MemoryBuffer => Buffer;
-
-		IList<ArraySegment<byte>> m_BufferList;
-		public IList<ArraySegment<byte>> BufferList {
-			get { return m_BufferList; }
-			set {
-				if (Buffer != null && value != null)
-					throw new ArgumentException ("Buffer and BufferList properties cannot both be non-null.");
-				m_BufferList = value;
-			}
-		}
-
 		public int BytesTransferred {
 			get;
 			private set;
 		}
 
-		public int Count {
-			get;
-			private set;
-		}
-
 		public bool DisconnectReuseSocket {
 			get;
 			set;
@@ -93,11 +71,6 @@ namespace System.Net.Sockets
 			private set;
 		}
 
-		public int Offset {
-			get;
-			private set;
-		}
-
 		public EndPoint RemoteEndPoint {
 			get { return remote_ep; }
 			set { remote_ep = value; }
@@ -157,6 +130,10 @@ namespace System.Net.Sockets
 			SendPacketsSendSize = -1;
 		}
 
+		internal SocketAsyncEventArgs (bool flowExecutionContext)
+		{
+		}
+
 		~SocketAsyncEventArgs ()
 		{
 			Dispose (false);
@@ -221,43 +198,6 @@ namespace System.Net.Sockets
 				handler (e.current_socket, e);
 		}
 
-		internal void CopyBufferFrom (SocketAsyncEventArgs source)
-		{
-			Buffer = source.Buffer;
-			Offset = source.Offset;
-			Count = source.Count;
-		}
-
-		public void SetBuffer (int offset, int count)
-		{
-			SetBuffer (Buffer, offset, count);
-		}
-
-		public void SetBuffer (byte[] buffer, int offset, int count)
-		{
-			if (buffer != null) {
-				if (BufferList != null)
-					throw new ArgumentException ("Buffer and BufferList properties cannot both be non-null.");
-				
-				int buflen = buffer.Length;
-				if (offset < 0 || (offset != 0 && offset >= buflen))
-					throw new ArgumentOutOfRangeException ("offset");
-
-				if (count < 0 || count > buflen - offset)
-					throw new ArgumentOutOfRangeException ("count");
-
-				Count = count;
-				Offset = offset;
-			}
-
-			Buffer = buffer;
-		}
-
-		public void SetBuffer(Memory<byte> buffer)
-		{
-			SetBuffer(buffer.ToArray(), 0, buffer.Length);
-		}
-
 		internal void StartOperationCommon (Socket socket)
 		{
 			current_socket = socket;
diff --git a/mcs/class/System/System.Net.Sockets/SocketAsyncResult.cs b/mcs/class/System/System.Net.Sockets/SocketAsyncResult.cs
index 4004e475854..99c940d4afe 100644
--- a/mcs/class/System/System.Net.Sockets/SocketAsyncResult.cs
+++ b/mcs/class/System/System.Net.Sockets/SocketAsyncResult.cs
@@ -43,7 +43,7 @@ namespace System.Net.Sockets
 		Exception DelayedException;
 
 		public EndPoint EndPoint;                 // Connect,ReceiveFrom,SendTo
-		public byte [] Buffer;                    // Receive,ReceiveFrom,Send,SendTo
+		public Memory<byte> Buffer;               // Receive,ReceiveFrom,Send,SendTo
 		public int Offset;                        // Receive,ReceiveFrom,Send,SendTo
 		public int Size;                          // Receive,ReceiveFrom,Send,SendTo
 		public SocketFlags SockFlags;             // Receive,ReceiveFrom,Send,SendTo
diff --git a/mcs/class/System/System.Net/ServicePoint.cs b/mcs/class/System/System.Net/ServicePoint.cs
index 4e59aa35372..2a501abcf82 100644
--- a/mcs/class/System/System.Net/ServicePoint.cs
+++ b/mcs/class/System/System.Net/ServicePoint.cs
@@ -392,11 +392,6 @@ namespace System.Net
 				return true;
 			}
 		}
-
-		internal Socket GetConnection(PooledStream PooledStream, object owner, bool async, out IPAddress address, ref Socket abortSocket, ref Socket abortSocket6)
-		{
-			throw new NotImplementedException ();
-		}
 	}
 }
 
diff --git a/mcs/class/System/System.Net/ServicePoint.platformnotsupported.cs b/mcs/class/System/System.Net/ServicePoint.platformnotsupported.cs
index d744a556442..3380a29c65b 100644
--- a/mcs/class/System/System.Net/ServicePoint.platformnotsupported.cs
+++ b/mcs/class/System/System.Net/ServicePoint.platformnotsupported.cs
@@ -117,11 +117,5 @@ namespace System.Net
 		public  X509Certificate ClientCertificate {
 			get { throw new PlatformNotSupportedException (EXCEPTION_MESSAGE); }
 		}
-
-		// For reference source
-		internal Socket GetConnection(PooledStream PooledStream, object owner, bool async, out IPAddress address, ref Socket abortSocket, ref Socket abortSocket6)
-		{
-			throw new NotImplementedException ();
-		}
 	}
 }
diff --git a/mcs/class/System/System.Net/WebConnection.cs b/mcs/class/System/System.Net/WebConnection.cs
index 5fd7e013a90..a6e6daf20ae 100644
--- a/mcs/class/System/System.Net/WebConnection.cs
+++ b/mcs/class/System/System.Net/WebConnection.cs
@@ -166,7 +166,21 @@ namespace System.Net
 				} else {
 					try {
 						operation.ThrowIfDisposed (cancellationToken);
-						await socket.ConnectAsync (remote).ConfigureAwait (false);
+
+						/*
+						 * Socket.Tasks.cs from CoreFX introduces a new internal
+						 * BeginConnect(EndPoint) overload, which will replace
+						 * the one we're using from SocketTaskExtensions.cs.
+						 *
+						 * Our implementation of Socket.BeginConnect() does not
+						 * invoke the callback when the request failed synchronously.
+						 *
+						 * Explicitly use our implementation from SocketTaskExtensions.cs here.
+						 */
+						await Task.Factory.FromAsync (
+							(targetEndPoint, callback, state) => ((Socket)state).BeginConnect (targetEndPoint, callback, state),
+							asyncResult => ((Socket)asyncResult.AsyncState).EndConnect (asyncResult),
+							remote, socket).ConfigureAwait (false);
 					} catch (ObjectDisposedException) {
 						throw;
 					} catch (Exception exc) {
diff --git a/mcs/class/System/System.csproj b/mcs/class/System/System.csproj
index 6223dd758bb..725075883ab 100644
--- a/mcs/class/System/System.csproj
+++ b/mcs/class/System/System.csproj
@@ -154,6 +154,7 @@
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\Mail\QuotedStringFormatReader.cs" />
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\Mail\WhitespaceReader.cs" />
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\NegotiationInfoClass.cs" />
+    <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\RangeValidationHelpers.cs" />
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\SecurityProtocol.cs" />
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\SecurityStatusPal.cs" />
     <Compile Include="..\..\..\external\corefx\src\Common\src\System\Net\Security\CertificateHelper.cs" />
@@ -453,6 +454,8 @@
     <Compile Include="..\..\..\external\corefx\src\System.Net.Primitives\src\System\Net\IPEndPoint.cs" />
     <Compile Include="..\..\..\external\corefx\src\System.Net.ServicePoint\src\System\Net\SecurityProtocolType.cs" />
     <Compile Include="..\..\..\external\corefx\src\System.Net.Sockets\src\System\Net\Sockets\MultipleConnectAsync.cs" />
+    <Compile Include="..\..\..\external\corefx\src\System.Net.Sockets\src\System\Net\Sockets\NetworkStream.cs" />
+    <Compile Include="..\..\..\external\corefx\src\System.Net.Sockets\src\System\Net\Sockets\Socket.Tasks.cs" />
     <Compile Include="..\..\..\external\corefx\src\System.Net.WebClient\src\System\Net\WebClient.cs" />
     <Compile Include="..\..\..\external\corefx\src\System.Net.WebSockets.Client\src\System\Net\WebSockets\ClientWebSocket.cs" />
     <Compile Include="..\..\..\external\corefx\src\System.Net.WebSockets.Client\src\System\Net\WebSockets\ClientWebSocketOptions.cs" />
@@ -699,7 +702,6 @@
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\IPProtectionLevel.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\LingerOption.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\MulticastOption.cs" />
-    <Compile Include="..\referencesource\System\net\System\Net\Sockets\NetworkStream.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\ProtocolFamily.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\ProtocolType.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\Sockets\SelectMode.cs" />
@@ -735,7 +737,6 @@
     <Compile Include="..\referencesource\System\net\System\Net\_LazyAsyncResult.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_LoggingObject.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_NetRes.cs" />
-    <Compile Include="..\referencesource\System\net\System\Net\_PooledStream.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_ProxyChain.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_ScatterGatherBuffers.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_Semaphore.cs" />
@@ -743,7 +744,6 @@
     <Compile Include="..\referencesource\System\net\System\Net\_TimerThread.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_WebProxyDataBuilder.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\_emptywebproxy.cs" />
-    <Compile Include="..\referencesource\System\net\System\Net\connectionpool.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\cookie.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\cookiecollection.cs" />
     <Compile Include="..\referencesource\System\net\System\Net\cookiecontainer.cs" />
@@ -1014,6 +1014,8 @@
     <Compile Include="System\UriTypeConverter.cs" />
     <Compile Include="corefx\CertificateRequest.cs" />
     <Compile Include="corefx\NameValueCollection.cs" />
+    <Compile Include="corefx\NetworkStream.cs" />
+    <Compile Include="corefx\SocketAsyncEventArgs.cs" />
     <Compile Include="corefx\StringDictionary.cs" />
     <Compile Include="corefx\X509SignatureGenerator.cs" />
     <Compile Include="corefx\ZLibNative.cs" />
diff --git a/mcs/class/System/System_xtest.dll.sources b/mcs/class/System/System_xtest.dll.sources
index 737cc2e6d5b..b96253754d5 100644
--- a/mcs/class/System/System_xtest.dll.sources
+++ b/mcs/class/System/System_xtest.dll.sources
@@ -1,6 +1,7 @@
 ../test-helpers/Configuration.Http.cs
 ../test-helpers/Configuration.Certificates.cs
 ../test-helpers/TestConfiguration.cs
+../test-helpers/NetworkStreamHelper.cs
 
 ../../../external/corefx/src/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs
 
@@ -96,6 +97,8 @@ System/RemoteExecutorTests.cs
 ../../../external/corefx/src/Common/tests/System/Net/Sockets/SocketTestServerAPM.cs
 ../../../external/corefx/src/Common/tests/System/Net/Sockets/SocketTestServerAsync.cs
 ../../../external/corefx/src/Common/tests/System/Net/Sockets/TestSettings.cs
+../../../external/corefx/src/System.Net.Sockets/tests/FunctionalTests/NetworkStreamTest.cs
+../../../external/corefx/src/System.Net.Sockets/tests/FunctionalTests/NetworkStreamTest.netcoreapp.cs
 #../../../external/corefx/src/System.Net.Sockets/tests/FunctionalTests/*.cs:NetworkStreamTest.netcoreapp.cs,SocketTestHelper.netcoreapp.cs,SendReceive.netcoreapp.cs,SocketAsyncEventArgsTest.cs,LoggingTest.cs,SendReceive.cs
 
 # System.Net.Primitives
diff --git a/mcs/class/System/common.sources b/mcs/class/System/common.sources
index de785afbbe1..eda8e8cdf70 100644
--- a/mcs/class/System/common.sources
+++ b/mcs/class/System/common.sources
@@ -587,7 +587,6 @@ ReferenceSources/Win32Exception.cs
 ../referencesource/System/net/System/Net/_NetRes.cs
 ../referencesource/System/net/System/Net/_LazyAsyncResult.cs
 ../referencesource/System/net/System/Net/_LoggingObject.cs
-../referencesource/System/net/System/Net/_PooledStream.cs
 ../referencesource/System/net/System/Net/_ProxyChain.cs
 ../referencesource/System/net/System/Net/_ScatterGatherBuffers.cs
 ../referencesource/System/net/System/Net/_Semaphore.cs
@@ -602,7 +601,6 @@ ReferenceSources/Win32Exception.cs
 ../referencesource/System/net/System/Net/cookiecollection.cs
 ../referencesource/System/net/System/Net/cookiecontainer.cs
 ../referencesource/System/net/System/Net/cookieexception.cs
-../referencesource/System/net/System/Net/connectionpool.cs
 ../referencesource/System/net/System/Net/DnsEndPoint.cs
 ../referencesource/System/net/System/Net/EndPoint.cs
 ../referencesource/System/net/System/Net/FtpStatusCode.cs
@@ -663,18 +661,20 @@ ReferenceSources/Win32Exception.cs
 ../referencesource/System/net/System/Net/SecureProtocols/AuthenticatedStream.cs
 ../referencesource/System/net/System/Net/SecureProtocols/NegotiateEnumTypes.cs
 
-../../../external/corefx/src/System.Net.Sockets/src/System/Net/Sockets/MultipleConnectAsync.cs
 ../referencesource/System/net/System/Net/Sockets/AddressFamily.cs
 ../referencesource/System/net/System/Net/Sockets/IOControlCode.cs
 ../referencesource/System/net/System/Net/Sockets/IPPacketInformation.cs
 ../referencesource/System/net/System/Net/Sockets/IPProtectionLevel.cs
 ../referencesource/System/net/System/Net/Sockets/LingerOption.cs
+../../../external/corefx/src/System.Net.Sockets/src/System/Net/Sockets/MultipleConnectAsync.cs
 ../referencesource/System/net/System/Net/Sockets/MulticastOption.cs
-../referencesource/System/net/System/Net/Sockets/NetworkStream.cs
+# ../referencesource/System/net/System/Net/Sockets/NetworkStream.cs
+../../../external/corefx/src/System.Net.Sockets/src/System/Net/Sockets/NetworkStream.cs
 ../referencesource/System/net/System/Net/Sockets/ProtocolFamily.cs
 ../referencesource/System/net/System/Net/Sockets/ProtocolType.cs
 ../referencesource/System/net/System/Net/Sockets/SelectMode.cs
 ../referencesource/System/net/System/Net/Sockets/Socket.cs
+../../../external/corefx/src/System.Net.Sockets/src/System/Net/Sockets/Socket.Tasks.cs
 ../referencesource/System/net/System/Net/Sockets/SocketErrors.cs
 ../referencesource/System/net/System/Net/Sockets/SocketFlags.cs
 ../referencesource/System/net/System/Net/Sockets/SocketInformation.cs
@@ -810,6 +810,7 @@ ReferenceSources/Win32Exception.cs
 ../../../external/corefx/src/Common/src/System/Net/ExceptionCheck.cs
 ../../../external/corefx/src/Common/src/System/Net/HttpStatusDescription.cs
 ../../../external/corefx/src/Common/src/System/Net/NegotiationInfoClass.cs
+../../../external/corefx/src/Common/src/System/Net/RangeValidationHelpers.cs
 ../../../external/corefx/src/Common/src/System/Net/SecurityProtocol.cs
 ../../../external/corefx/src/Common/src/System/Net/SecurityStatusPal.cs
 ../../../external/corefx/src/Common/src/System/Net/UriScheme.cs
@@ -914,3 +915,4 @@ ReferenceSources/Win32Exception.cs
 
 ../../../external/corefx/src/System.Text.RegularExpressions/src/System/Collections/Generic/ValueListBuilder.Pop.cs
 ../../../external/corefx/src/System.Text.RegularExpressions/src/System/Collections/HashtableExtensions.cs
+
diff --git a/mcs/class/System/corefx/NetworkStream.cs b/mcs/class/System/corefx/NetworkStream.cs
new file mode 100644
index 00000000000..d18f8e51683
--- /dev/null
+++ b/mcs/class/System/corefx/NetworkStream.cs
@@ -0,0 +1,18 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace System.Net.Sockets
+{
+	partial class NetworkStream
+	{
+		internal Socket InternalSocket {
+			get {
+				Socket chkSocket = _streamSocket;
+				if (_cleanedUp || chkSocket == null)
+					throw new ObjectDisposedException (GetType ().FullName);
+
+				return chkSocket;
+			}
+		}
+	}
+}
diff --git a/mcs/class/System/corefx/SocketAsyncEventArgs.cs b/mcs/class/System/corefx/SocketAsyncEventArgs.cs
new file mode 100644
index 00000000000..ddc8793bbf8
--- /dev/null
+++ b/mcs/class/System/corefx/SocketAsyncEventArgs.cs
@@ -0,0 +1,169 @@
+using System;
+using System.Diagnostics;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using System.Reflection;
+using System.Security;
+using System.Threading;
+
+namespace System.Net.Sockets
+{
+    partial class SocketAsyncEventArgs
+    {
+        // Single buffer.
+        private Memory<byte> _buffer;
+        private int _offset;
+        private int _count;
+        private bool _bufferIsExplicitArray;
+
+        // BufferList property variables.
+        private IList<ArraySegment<byte>> _bufferList;
+        private List<ArraySegment<byte>> _bufferListInternal;
+
+        public byte[] Buffer
+        {
+            get
+            {
+                if (_bufferIsExplicitArray)
+                {
+                    bool success = MemoryMarshal.TryGetArray(_buffer, out ArraySegment<byte> arraySegment);
+                    Debug.Assert(success);
+                    return arraySegment.Array;
+                }
+
+                return null;
+            }
+        }
+
+        public Memory<byte> MemoryBuffer => _buffer;
+
+        public int Offset => _offset;
+
+        public int Count => _count;
+
+        // NOTE: this property is mutually exclusive with Buffer.
+        // Setting this property with an existing non-null Buffer will throw.
+        public IList<ArraySegment<byte>> BufferList
+        {
+            get { return _bufferList; }
+            set
+            {
+                if (value != null)
+                {
+                    if (!_buffer.Equals(default))
+                    {
+                        // Can't have both set
+                        throw new ArgumentException(SR.Format(SR.net_ambiguousbuffers, nameof(Buffer)));
+                    }
+
+                    // Copy the user-provided list into our internal buffer list,
+                    // so that we are not affected by subsequent changes to the list.
+                    // We reuse the existing list so that we can avoid reallocation when possible.
+                    int bufferCount = value.Count;
+                    if (_bufferListInternal == null)
+                    {
+                        _bufferListInternal = new List<ArraySegment<byte>>(bufferCount);
+                    }
+                    else
+                    {
+                        _bufferListInternal.Clear();
+                    }
+
+                    for (int i = 0; i < bufferCount; i++)
+                    {
+                        ArraySegment<byte> buffer = value[i];
+                        RangeValidationHelpers.ValidateSegment(buffer);
+                        _bufferListInternal.Add(buffer);
+                    }
+                }
+                else
+                {
+                    _bufferListInternal?.Clear();
+                }
+
+                _bufferList = value;
+            }
+        }
+
+        public void SetBuffer(int offset, int count)
+        {
+            if (!_buffer.Equals(default))
+            {
+                if ((uint)offset > _buffer.Length)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(offset));
+                }
+                if ((uint)count > (_buffer.Length - offset))
+                {
+                    throw new ArgumentOutOfRangeException(nameof(count));
+                }
+                if (!_bufferIsExplicitArray)
+                {
+                    throw new InvalidOperationException(SR.InvalidOperation_BufferNotExplicitArray);
+                }
+
+                _offset = offset;
+                _count = count;
+            }
+        }
+
+        internal void CopyBufferFrom(SocketAsyncEventArgs source)
+        {
+            _buffer = source._buffer;
+            _offset = source._offset;
+            _count = source._count;
+            _bufferIsExplicitArray = source._bufferIsExplicitArray;
+        }
+
+        public void SetBuffer(byte[] buffer, int offset, int count)
+        {
+            if (buffer == null)
+            {
+                // Clear out existing buffer.
+                _buffer = default;
+                _offset = 0;
+                _count = 0;
+                _bufferIsExplicitArray = false;
+            }
+            else
+            {
+                // Can't have both Buffer and BufferList.
+                if (_bufferList != null)
+                {
+                    throw new ArgumentException(SR.Format(SR.net_ambiguousbuffers, nameof(BufferList)));
+                }
+
+                // Offset and count can't be negative and the
+                // combination must be in bounds of the array.
+                if ((uint)offset > buffer.Length)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(offset));
+                }
+                if ((uint)count > (buffer.Length - offset))
+                {
+                    throw new ArgumentOutOfRangeException(nameof(count));
+                }
+
+                _buffer = buffer;
+                _offset = offset;
+                _count = count;
+                _bufferIsExplicitArray = true;
+            }
+        }
+
+        public void SetBuffer(Memory<byte> buffer)
+        {
+            if (buffer.Length != 0 && _bufferList != null)
+            {
+                throw new ArgumentException(SR.Format(SR.net_ambiguousbuffers, nameof(BufferList)));
+            }
+
+            _buffer = buffer;
+            _offset = 0;
+            _count = buffer.Length;
+            _bufferIsExplicitArray = false;
+        }
+
+        internal bool HasMultipleBuffers => _bufferList != null;
+    }
+}