[C#] Verify the file identifier instead of skipping the check

net/FlatBuffers/FlatBufferVerify.cs

@@ -434,7 +434,7 @@ private bool CheckIndirectOffset(uint pos)
     private bool CheckBufferFromStart(string identifier, uint startPos, VerifyTableAction verifyAction)
     {
       if ((identifier != null) &&
-          (identifier.Length == 0) &&
+          (identifier.Length != 0) &&
           ((verifier_buffer.Length < (SIZE_U_OFFSET + FILE_IDENTIFIER_LENGTH)) || (!BufferHasIdentifier(verifier_buffer, startPos, identifier))))
       {
         return false;

tests/FlatBuffers.Test/FlatBufferVerifyIdentifierTests.cs

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2026 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System;
+
+namespace Google.FlatBuffers.Test
+{
+    [FlatBuffersTestClass]
+    public class FlatBufferVerifyIdentifierTests
+    {
+        // The smallest real table verify action: exactly what generated code
+        // emits for a table without fields.
+        private static bool VerifyEmptyTable(Verifier verifier, uint tablePos)
+        {
+            return verifier.VerifyTableStart(tablePos) && verifier.VerifyTableEnd(tablePos);
+        }
+
+        private static byte[] BuildBuffer(string identifier)
+        {
+            var fbb = new FlatBufferBuilder(64);
+            fbb.StartTable(0);
+            int root = fbb.EndTable();
+            if (identifier == null)
+            {
+                fbb.Finish(root);
+            }
+            else
+            {
+                fbb.Finish(root, identifier);
+            }
+            return fbb.SizedByteArray();
+        }
+
+        private static bool Verify(byte[] buffer, string identifier)
+        {
+            var verifier = new Verifier(new ByteBuffer(buffer));
+            return verifier.VerifyBuffer(identifier, false, VerifyEmptyTable);
+        }
+
+        [FlatBuffersTestMethod]
+        public void VerifyBufferAcceptsMatchingIdentifier()
+        {
+            Assert.IsTrue(Verify(BuildBuffer("NULL"), "NULL"));
+        }
+
+        [FlatBuffersTestMethod]
+        public void VerifyBufferRejectsMismatchedIdentifier()
+        {
+            Assert.IsFalse(Verify(BuildBuffer("NULL"), "ZZZZ"));
+        }
+
+        [FlatBuffersTestMethod]
+        public void VerifyBufferRejectsMissingIdentifier()
+        {
+            Assert.IsFalse(Verify(BuildBuffer(null), "NULL"));
+        }
+
+        [FlatBuffersTestMethod]
+        public void VerifyBufferSkipsCheckForEmptyIdentifier()
+        {
+            // Documented contract in FlatBufferVerify.cs: "When empty identifier
+            // is provided the identifier validation is skipped." flatc emits
+            // VerifyBuffer("", ...) for schemas that declare no file_identifier.
+            Assert.IsTrue(Verify(BuildBuffer(null), ""));
+            Assert.IsTrue(Verify(BuildBuffer("NULL"), ""));
+        }
+    }
+}

tests/FlatBuffers.Test/FlatBuffersExampleTests.cs

@@ -1221,7 +1221,7 @@ public void TestVerifyingUnions()
             Monster.AddTest(fbb, monster_inner.Value);
             Monster.AddTestType(fbb, Any.Monster);
             var monster_outer = Monster.EndMonster(fbb);
-            fbb.Finish(monster_outer.Value);
+            Monster.FinishMonsterBuffer(fbb, monster_outer);
             var bytes = fbb.SizedByteArray();
             var bytes_to_corrupt_inner_name = fbb.SizedByteArray();
             var bytes_to_corrupt_outer_name = fbb.SizedByteArray();