Update: Moving files from Internals to Sockets section in Common

src/libraries/Common/src/System/Net/Sockets/SocketExceptionFactory.Unix.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Net.Sockets
+{
+    internal static partial class SocketExceptionFactory
+    {
+        public static SocketException CreateSocketException(int socketError, EndPoint? endPoint)
+        {
+            int nativeErr = (int)socketError;
+
+            // If an interop error was not found, then don't invoke Info().RawErrno as that will fail with assert.
+            if (SocketErrorPal.TryGetNativeErrorForSocketError(error, out Interop.Error interopErr))
+            {
+                nativeErr = interopErr.Info().RawErrno;
+            }
+
+            return new SocketException(socketError, CreateMessage(nativeErr, endPoint));
+        }
+    }
+}

src/libraries/Common/src/System/Net/Internals/SocketExceptionFactory.cs → src/libraries/Common/src/System/Net/Sockets/SocketExceptionFactory.Windows.cs

@@ -1,15 +1,14 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Net.Sockets;
-
-namespace System.Net.Internals
+namespace System.Net.Sockets
 {
     internal static partial class SocketExceptionFactory
     {
         public static SocketException CreateSocketException(int socketError, EndPoint? endPoint)
         {
-            return new SocketException(socketError, endPoint?.ToString());
+            // Windows directly maps socketError to native error code.
+            return new SocketException(socketError, CreateMessage(socketError, endPoint));
         }
     }
 }

src/libraries/Common/src/System/Net/Sockets/SocketExceptionFactory.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+
+namespace System.Net.Sockets
+{
+    internal static partial class SocketExceptionFactory
+    {
+        private static string CreateMessage(int nativeSocketError, EndPoint? endPoint)
+        {
+            string message = Marshal.GetPInvokeErrorMessage(nativeSocketError);
+            return endPoint == null ? message : message + " " + endPoint.ToString();
+        }
+    }
+}

src/libraries/System.Net.Primitives/tests/FunctionalTests/SocketExceptionTest.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Net.Sockets;
+using System.Runtime.InteropServices;
 using Xunit;
 
 namespace System.Net.Primitives.Functional.Tests
@@ -45,7 +46,7 @@ public static void Create_SocketConnectException_Success()
                 }
                 catch(SocketException ex)
                 {
-                    Assert.Equal(ep.ToString(), ex.Message);
+                    Assert.Contains(Marshal.GetPInvokeErrorMessage(ex.NativeErrorCode) + " " + ep.ToString(), ex.Message);
                 }
             }
         }

src/libraries/System.Net.Sockets/src/System.Net.Sockets.csproj

@@ -78,8 +78,8 @@
              Link="Common\System\Net\Internals\IPEndPointExtensions.cs" />
     <Compile Include="$(CommonPath)System\Net\Internals\IPAddressExtensions.cs"
              Link="Common\System\Net\Internals\IPAddressExtensions.cs" />
-    <Compile Include="$(CommonPath)System\Net\Internals\SocketExceptionFactory.cs"
-             Link="Common\System\Net\Internals\SocketExceptionFactory.cs" />
+    <Compile Include="$(CommonPath)System\Net\Sockets\SocketExceptionFactory.cs"
+             Link="Common\System\Net\Sockets\SocketExceptionFactory.cs" />
     <Compile Include="$(CommonPath)System\Net\Sockets\ProtocolFamily.cs"
              Link="Common\System\Net\Sockets\ProtocolFamily.cs" />
     <Compile Include="$(CommonPath)System\Net\Sockets\ProtocolType.cs"
@@ -100,6 +100,8 @@
              Link="Common\System\Net\SocketAddressPal.Windows.cs" />
     <Compile Include="$(CommonPath)System\Net\SocketProtocolSupportPal.Windows.cs"
              Link="Common\System\Net\SocketProtocolSupportPal.Windows" />
+    <Compile Include="$(CommonPath)System\Net\Sockets\SocketExceptionFactory.Windows.cs"
+             Link="Common\System\Net\Sockets\SocketExceptionFactory.Windows.cs" />
     <!-- Interop -->
     <Compile Include="$(CommonPath)Interop\Windows\Interop.Libraries.cs"
              Link="Common\Interop\Windows\Interop.Libraries.cs" />
@@ -278,6 +280,8 @@
              Link="Common\Interop\Unix\System.Native\Interop.Pipe.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Write.cs"
              Link="Common\Interop\Unix\System.Native\Interop.Write.cs" />
+    <Compile Include="$(CommonPath)System\Net\Sockets\SocketExceptionFactory.Unix.cs"
+             Link="Common\System\Net\Sockets\SocketExceptionFactory.Unix.cs" />
   </ItemGroup>
   <ItemGroup>
     <Reference Include="Microsoft.Win32.Primitives" />