Fix field count encoding in DescribedList and add tests for full field count compliance

src/Types/DescribedList.cs

@@ -401,22 +401,17 @@ internal override void DecodeValue(ByteBuffer buffer)
 
         internal override void EncodeValue(ByteBuffer buffer)
         {
-            if (this.fields == 0)
+            if (this.fieldCount == 0)
             {
                 AmqpBitConverter.WriteUByte(buffer, FormatCode.List0);
             }
             else
             {
-                // Count non-null fields by removing leading zeros
-                int count = 0;
-                int temp = this.fields;
-                while (temp > 0)
-                {
-                    count++;
-                    temp <<= 1;
-                }
-
-                count = 32 - count;
+                // Always encode all defined fields so receivers that access fields by fixed
+                // numeric index (e.g. Python _pyamqp) get the full-length list regardless of
+                // which fields are set. This matches real broker behaviour and satisfies
+                // AMQP 1.0 spec section 1.4 (trailing nulls are permitted).
+                int count = this.fieldCount;
                 int pos = buffer.WritePos;
                 AmqpBitConverter.WriteUByte(buffer, 0);
                 AmqpBitConverter.WriteULong(buffer, 0);

test/Common/EncoderTests.cs

@@ -74,5 +74,61 @@ public void EncoderNestedNestedListTest()
             Assert.AreEqual(1, subList.Count);
             Assert.AreEqual("test", subList[0]);
         }
+
+        // Verify that every AMQP 1.0 performative is encoded with its full field count even
+        // when only mandatory fields are set. Receivers that access fields by fixed numeric
+        // index (e.g. Python _pyamqp) must see the complete list length.
+        [TestMethod]
+        public void PerformativesEncodeFullFieldCount()
+        {
+            // (performative, expected field count per AMQP 1.0 spec)
+            var cases = new (global::Amqp.Types.DescribedList frame, int expectedCount)[]
+            {
+                (new global::Amqp.Framing.Open { ContainerId = "test" }, 10),
+                (new global::Amqp.Framing.Begin { NextOutgoingId = 0, IncomingWindow = 100, OutgoingWindow = 100 }, 8),
+                (new global::Amqp.Framing.Attach { LinkName = "test", Handle = 0, Role = true }, 14),
+                (new global::Amqp.Framing.Flow(), 11),
+                (new global::Amqp.Framing.Transfer { Handle = 0 }, 11),
+                (new global::Amqp.Framing.Dispose { Role = false }, 6),
+                (new global::Amqp.Framing.Detach { Handle = 0 }, 3),
+                (new global::Amqp.Framing.End(), 1),
+                (new global::Amqp.Framing.Close(), 1),
+            };
+
+            foreach (var (frame, expectedCount) in cases)
+            {
+                var buffer = new ByteBuffer(512, true);
+                frame.Encode(buffer);
+
+                // Skip described-type prefix: 0x00 + ulong format code + descriptor value
+                AmqpBitConverter.ReadUByte(buffer); // 0x00 (described type)
+                byte ulongFc = AmqpBitConverter.ReadUByte(buffer);
+                if (ulongFc == 0x53) // smallulong
+                    AmqpBitConverter.ReadUByte(buffer);
+                else // 0x80 ulong
+                    AmqpBitConverter.ReadULong(buffer);
+
+                // Read list header and extract count
+                byte listFc = AmqpBitConverter.ReadUByte(buffer);
+                int count;
+                if (listFc == 0x45) // list0 — empty
+                {
+                    count = 0;
+                }
+                else if (listFc == 0xc0) // list8 — size(1) + count(1)
+                {
+                    AmqpBitConverter.ReadUByte(buffer); // size
+                    count = AmqpBitConverter.ReadUByte(buffer);
+                }
+                else // list32 (0xd0) — size(4) + count(4)
+                {
+                    AmqpBitConverter.ReadUInt(buffer); // size
+                    count = (int)AmqpBitConverter.ReadUInt(buffer);
+                }
+
+                Assert.AreEqual(expectedCount, count,
+                    $"{frame.GetType().Name} expected {expectedCount} fields but encoded {count}");
+            }
+        }
     }
 }