Support Discriminator for ObjectFormat

- Use semantig tag 39 to tag discriminator when CborObjectFormat is Array
- Discriminator MemberIndex is always 0 when CborObjectFormat is Array or IntKeyMap

src/Dahomey.Cbor.Tests/ObjectFormatTests.cs

@@ -1,6 +1,8 @@
 using Dahomey.Cbor.Attributes;
 using System.Reflection;
+using System.Xml.Linq;
 using Xunit;
+using static Dahomey.Cbor.Tests.DiscriminatorTests;
 
 namespace Dahomey.Cbor.Tests
 {
@@ -240,5 +242,123 @@ public void ConstructorByAttribute(CborObjectFormat objectFormat, string hexBuff
             Assert.Equal(expectedName, obj.Name);
             Assert.Equal(expectedAge, obj.Age);
         }
+
+        [CborObjectFormat(CborObjectFormat.IntKeyMap)]
+        public class WithId
+        {
+            [CborProperty(1)]
+            public int Id { get; set; }
+        }
+
+        [CborDiscriminator("Person4")]
+        [CborObjectFormat(CborObjectFormat.IntKeyMap)]
+        public class Person4 : WithId
+        {
+            [CborProperty(2)]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public void ReadIntKeyMapWithDiscrimator()
+        {
+            CborOptions options = new CborOptions();
+            options.Registry.DiscriminatorConventionRegistry.RegisterType<Person4>();
+
+            const string hexBuffer = "A30067506572736F6E34010C0263666F6F"; // {0: "Person4", 1: 12, 2: "foo"}
+            WithId obj = Helper.Read<WithId>(hexBuffer, options);
+            Assert.NotNull(obj);
+            Person4 person = Assert.IsType<Person4>(obj);
+            Assert.Equal(12, person.Id);
+            Assert.Equal("foo", person.Name);
+
+            const string hexBuffer2 = "A2010C0263666F6F"; // {1: 12, 2: "foo"}
+            Person4 obj2 = Helper.Read<Person4>(hexBuffer2, options); // no inheritance, no discriminator written
+            Assert.Equal(12, obj2.Id);
+            Assert.Equal("foo", obj2.Name);
+        }
+
+        [Fact]
+        public void WriteIntKeyMapWithDiscrimator()
+        {
+            CborOptions options = new CborOptions();
+            options.Registry.DiscriminatorConventionRegistry.RegisterType<Person4>();
+
+            WithId person = new Person4
+            {
+                Id = 12,
+                Name = "foo"
+            };
+
+            const string hexBuffer = "A30067506572736F6E34010C0263666F6F"; // {0: "Person4", 1: 12, 2: "foo"}
+            Helper.TestWrite(person, hexBuffer, null, options);
+
+            Person4 person2 = new Person4 // no inheritance, no discriminator written
+            {
+                Id = 12,
+                Name = "foo"
+            };
+
+            const string hexBuffer2 = "A2010C0263666F6F"; // {1: 12, 2: "foo"}
+            Helper.TestWrite(person2, hexBuffer2, null, options);
+        }
+
+        [CborObjectFormat(CborObjectFormat.Array)]
+        public class WithId2
+        {
+            [CborProperty(1)]
+            public int Id { get; set; }
+        }
+
+        [CborDiscriminator("Person4")]
+        [CborObjectFormat(CborObjectFormat.Array)]
+        public class Person5 : WithId2
+        {
+            [CborProperty(2)]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public void ReadArrayWithDiscrimator()
+        {
+            CborOptions options = new CborOptions();
+            options.Registry.DiscriminatorConventionRegistry.RegisterType<Person5>();
+
+            const string hexBuffer = "83D82767506572736F6E340C63666F6F"; // [39("Person4"), 12, "foo"]
+            WithId2 obj = Helper.Read<WithId2>(hexBuffer, options);
+            Assert.NotNull(obj);
+            Person5 person = Assert.IsType<Person5>(obj);
+            Assert.Equal(12, person.Id);
+            Assert.Equal("foo", person.Name);
+
+            const string hexBuffer2 = "820C63666F6F"; // [12, "foo"]
+            Person5 obj2 = Helper.Read<Person5>(hexBuffer2, options); // no inheritance, no discriminator written
+            Assert.Equal(12, obj2.Id);
+            Assert.Equal("foo", obj2.Name);
+        }
+
+        [Fact]
+        public void WriteArrayWithDiscrimator()
+        {
+            CborOptions options = new CborOptions();
+            options.Registry.DiscriminatorConventionRegistry.RegisterType<Person5>();
+
+            WithId2 person = new Person5
+            {
+                Id = 12,
+                Name = "foo"
+            };
+
+            const string hexBuffer = "83D82767506572736F6E340C63666F6F"; // [39("Person4"), 12, "foo"]
+            Helper.TestWrite(person, hexBuffer, null, options);
+
+            Person5 person2 = new Person5 // no inheritance, no discriminator written
+            {
+                Id = 12,
+                Name = "foo"
+            };
+
+            const string hexBuffer2 = "820C63666F6F"; // [12, "foo"]
+            Helper.TestWrite(person2, hexBuffer2, null, options);
+        }
     }
 }

src/Dahomey.Cbor/CborOptions.cs

@@ -53,8 +53,13 @@ public class CborOptions
         public CborObjectFormat ObjectFormat { get; set; } = CborObjectFormat.StringKeyMap;
         public LengthMode ArrayLengthMode { get; set; } = LengthMode.DefiniteLength;
         public LengthMode MapLengthMode { get; set; } = LengthMode.DefiniteLength;
+        /// <summary>
+        /// Semantic Tag to check if the discriminator is present when ObjectFormat is Array
+        /// </summary>
+        /// Default value is 39 (see: https://github.com/lucas-clemente/cbor-specs/blob/master/id.md)
+        public ulong DiscriminatorSemanticTag { get; set; } = 39;
 
-        public CborOptions()
+    public CborOptions()
         {
             Registry = new SerializationRegistry(this);
         }

src/Dahomey.Cbor/Serialization/Converters/Mappings/DiscriminatorMapping.cs

@@ -13,21 +13,14 @@ public interface IDiscriminatorMapping : IMemberMapping
     public class DiscriminatorMapping<T> : IDiscriminatorMapping
     {
         private bool _isInitialized = false;
+        private readonly CborOptions _options;
         private readonly DiscriminatorConventionRegistry _discriminatorConventionRegistry;
         private readonly IObjectMapping _objectMapping;
         private string? _memberName = null;
-        private int? _memberIndex = null;
 
         public MemberInfo? MemberInfo => null;
         public Type MemberType => throw new NotSupportedException();
-        public int? MemberIndex
-        {
-            get
-            {
-                EnsureInitialize();
-                return _memberIndex;
-            }
-        }
+        public int? MemberIndex => 0; // discriminator gets always the first index
         public string? MemberName
         {
             get
@@ -45,10 +38,10 @@ public class DiscriminatorMapping<T> : IDiscriminatorMapping
         public LengthMode LengthMode => LengthMode.Default;
         public RequirementPolicy RequirementPolicy => RequirementPolicy.Never;
 
-        public DiscriminatorMapping(DiscriminatorConventionRegistry discriminatorConventionRegistry, 
-            IObjectMapping objectMapping)
+        public DiscriminatorMapping(CborOptions options, IObjectMapping objectMapping)
         {
-            _discriminatorConventionRegistry = discriminatorConventionRegistry;
+            _options = options;
+            _discriminatorConventionRegistry = options.Registry.DiscriminatorConventionRegistry;
             _objectMapping = objectMapping;
         }
 
@@ -79,8 +72,10 @@ public IMemberConverter GenerateMemberConverter()
                 throw new CborException($"Cannot find a discriminator convention for type {_objectMapping.ObjectType}");
             }
 
+            int? memberIndex = _objectMapping.ObjectFormat == CborObjectFormat.Array ? 0 : null;
+
             IMemberConverter memberConverter = new DiscriminatorMemberConverter<T>(
-                discriminatorConvention, _objectMapping.DiscriminatorPolicy);
+                _options, discriminatorConvention, _objectMapping.DiscriminatorPolicy, _objectMapping.ObjectFormat);
 
             return memberConverter;
         }

src/Dahomey.Cbor/Serialization/Converters/Mappings/ObjectMapping.cs

@@ -81,7 +81,7 @@ public ObjectMapping<T> SetDiscriminator(object discriminator)
                 && _registry.DiscriminatorConventionRegistry.AnyConvention()
                 && (_memberMappings.Count == 0 || _memberMappings[0] is not DiscriminatorMapping<T>))
             {
-                DiscriminatorMapping<T> memberMapping = new DiscriminatorMapping<T>(_registry.DiscriminatorConventionRegistry, this);
+                DiscriminatorMapping<T> memberMapping = new DiscriminatorMapping<T>(_options, this);
                 _memberMappings.Insert(0, memberMapping);
             }
 
@@ -238,7 +238,7 @@ public ObjectMapping<T> SetDiscriminatorPolicy(CborDiscriminatorPolicy discrimin
                 && _registry.DiscriminatorConventionRegistry.AnyConvention()
                 && (_memberMappings.Count == 0 || _memberMappings[0] is not DiscriminatorMapping<T>))
             {
-                DiscriminatorMapping<T> memberMapping = new DiscriminatorMapping<T>(_registry.DiscriminatorConventionRegistry, this);
+                DiscriminatorMapping<T> memberMapping = new DiscriminatorMapping<T>(_options, this);
                 _memberMappings.Insert(0, memberMapping);
             }
 
@@ -366,17 +366,18 @@ private void ValidateMemberNamesAndindexes()
                             throw new CborException($"exepcting all fields/properties to get a member index in class/struct {ObjectType.Name}");
                         }
 
-                        _memberMappings = _memberMappings
-                            .OrderBy(m => m.MemberIndex)
-                            .ToList();
+                        bool indexDuplicates = _memberMappings
+                            .GroupBy(x => x.MemberIndex)
+                            .Any(g => g.Count() > 1);
 
-                        for (int i = 0; i < _memberMappings.Count; i++)
+                        if (indexDuplicates)
                         {
-                            if (_memberMappings[i].MemberIndex != i)
-                            {
-                                throw new CborException($"class/struct {ObjectType.Name} MemberIndexes must follow one another with no holes");
-                            }
+                            throw new CborException($"class/struct {ObjectType.Name} holds duplicated MemberIndex fields/properties");
                         }
+
+                        _memberMappings = _memberMappings
+                            .OrderBy(m => m.MemberIndex)
+                            .ToList();
                     }
                     break;
             }

src/Dahomey.Cbor/Serialization/Converters/MemberConverter.cs

@@ -373,16 +373,22 @@ public bool ShouldSerialize(ref T instance, Type declaredType)
 
     public class DiscriminatorMemberConverter<T> : IMemberConverter
     {
+        private readonly CborOptions _options;
         private readonly IDiscriminatorConvention _discriminatorConvention;
         private readonly CborDiscriminatorPolicy _discriminatorPolicy;
+        private readonly CborObjectFormat _objectFormat;
         private readonly ReadOnlyMemory<byte> _memberName;
 
         public DiscriminatorMemberConverter(
-            IDiscriminatorConvention discriminatorConvention, 
-            CborDiscriminatorPolicy discriminatorPolicy)
+            CborOptions options,
+            IDiscriminatorConvention discriminatorConvention,
+            CborDiscriminatorPolicy discriminatorPolicy,
+            CborObjectFormat objectFormat)
         {
+            _options = options;
             _discriminatorConvention = discriminatorConvention;
             _discriminatorPolicy = discriminatorPolicy;
+            _objectFormat = objectFormat;
 
             if (discriminatorConvention != null)
             {
@@ -391,7 +397,7 @@ public class DiscriminatorMemberConverter<T> : IMemberConverter
         }
 
         public ReadOnlySpan<byte> MemberName => _memberName.Span;
-        public int? MemberIndex => throw new NotSupportedException();
+        public int? MemberIndex => 0; // discriminator gets always the first index
         public bool IgnoreIfDefault => false;
         public RequirementPolicy RequirementPolicy => RequirementPolicy.Never;
 
@@ -426,6 +432,11 @@ public bool ShouldSerialize(object obj, Type declaredType, CborOptions options)
 
         public void Write(ref CborWriter writer, object obj)
         {
+            if (_objectFormat == CborObjectFormat.Array)
+            {
+                // we need a Semantic Tag to check if the discriminator is present
+                writer.WriteSemanticTag(_options.DiscriminatorSemanticTag);
+            }
             _discriminatorConvention.WriteDiscriminator(ref writer, obj.GetType());
         }
     }