From 16a69dfa7e03366538f3bcfe72455f4c41f9bca4 Mon Sep 17 00:00:00 2001
From: Laveesh Bansal <laveeshbansal@gmail.com>
Date: Fri, 2 Jan 2026 22:12:05 -0800
Subject: [PATCH] Socket.SendFile throws InvalidOperationException when
 Blocking=false

The behavior of SendFile on non-blocking sockets is undefined (Windows blocks anyway, Linux may send partial data), so we now throw InvalidOperationException early.
---
 .../src/System/Net/Sockets/Socket.cs                 |  7 +++++++
 .../tests/FunctionalTests/SendFile.cs                | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs b/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs
index 4f5598eac020d0..3b62533e7a82e6 100644
--- a/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs
+++ b/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);
diff --git a/src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs b/src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs
index f85d68d05c8b13..7788a14530347b 100644
--- a/src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs
+++ b/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)]