Fix: Socket.SendFile throws when Blocking=false

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

@@ -1325,6 +1325,13 @@ public void SendFile(string? fileName, ReadOnlySpan<byte> preBuffer, ReadOnlySpa
 
             ThrowIfDisposed();
 
+            // SendFile is not supported on non-blocking sockets.
+            // ValidateBlockingMode() below checks for async mismatch; this checks explicit non-blocking.
+            if (!Blocking)
+            {
+                throw new InvalidOperationException(SR.net_sockets_blocking);
+            }
+
             if (!IsConnectionOriented || !Connected)
             {
                 throw new NotSupportedException(SR.net_notconnected);

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

@@ -37,6 +37,18 @@ public async Task NotConnected_ThrowsNotSupportedException()
             await Assert.ThrowsAsync<NotSupportedException>(() => SendFileAsync(s, null, null, null, TransmitFileOptions.UseDefaultWorkerThread));
         }
 
+        [Fact]
+        public void SendFile_NonBlockingSocket_ThrowsInvalidOperationException()
+        {
+            (Socket client, Socket server) = SocketTestExtensions.CreateConnectedSocketPair();
+            using (client)
+            using (server)
+            {
+                client.Blocking = false;
+                Assert.Throws<InvalidOperationException>(() => client.SendFile(null));
+            }
+        }
+
         [Theory]
         [InlineData(false)]
         [InlineData(true)]