Fix CborWriter.Reset.

src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Writer/CborWriter.cs

@@ -110,7 +110,7 @@ public void Reset()
                 _offset = 0;
                 _nestedDataItems?.Clear();
                 _currentMajorType = null;
-                _definiteLength = null;
+                _definiteLength = AllowMultipleRootLevelValues ? null : (int?)1;
                 _itemsWritten = 0;
                 _frameOffset = 0;
                 _isTagContext = false;
@@ -120,6 +120,7 @@ public void Reset()
                 _keysRequireSorting = false;
                 _keyValuePairEncodingRanges?.Clear();
                 _keyEncodingRanges?.Clear();
+                _currentIndefiniteLengthStringRanges?.Clear();
             }
         }
 

src/libraries/System.Formats.Cbor/tests/Writer/CborWriterTests.cs

@@ -123,6 +123,33 @@ public static void Reset_NonTrivialWriter_HappyPath()
             Assert.Equal(new byte[] { 0x18, 0x2a }, writer.Encode());
         }
 
+        [Fact]
+        public static void Reset_PreservesMultipleRootLevels()
+        {
+            var writer = new CborWriter(allowMultipleRootLevelValues: false);
+            writer.WriteBoolean(true);
+            writer.Reset();
+            writer.WriteInt32(6);
+            Assert.Throws<InvalidOperationException>(() => writer.WriteInt32(7));
+        }
+
+        [Fact]
+        public static void Reset_ClearsIndefiniteLengthRanges()
+        {
+            var writer = new CborWriter(convertIndefiniteLengthEncodings: true);
+
+            writer.WriteStartIndefiniteLengthByteString();
+            writer.WriteByteString([1, 2, 3]);
+            writer.Reset();
+
+            writer.WriteStartIndefiniteLengthByteString();
+            writer.WriteByteString([1, 2, 3]);
+            writer.WriteEndIndefiniteLengthByteString();
+            ReadOnlySpan<byte> encoded = writer.Encode();
+            ReadOnlySpan<byte> expected = [0x43, 0x01, 0x02, 0x03];
+            AssertExtensions.SequenceEqual(expected, encoded);
+        }
+
         [Fact]
         public static void ConformanceMode_DefaultValue_ShouldEqualStrict()
         {