Throw an exception on duplicate keys

YamlDotNet.Test/Serialization/DeserializerTest.cs

@@ -24,6 +24,7 @@
 using System.Linq;
 using FluentAssertions;
 using Xunit;
+using YamlDotNet.Core;
 using YamlDotNet.Serialization;
 using YamlDotNet.Serialization.NamingConventions;
 
@@ -216,7 +217,6 @@ public void NewLinesInKeys()
             Assert.Equal($"value\na\nb", dictionary.First().Value);
         }
 
-
         [Theory]
         [InlineData(".nan", System.Single.NaN)]
         [InlineData(".NaN", System.Single.NaN)]
@@ -283,6 +283,41 @@ public void DeserializeScalarEdgeCases(IConvertible value, Type type)
             result.Should().Be(value);
         }
 
+        [Fact]
+        public void DeserializeWithDuplicateKeyChecking_YamlWithDuplicateKeys_ThrowsYamlException()
+        {
+            var yaml = @"
+name: Jack
+momentOfBirth: 1983-04-21T20:21:03.0041599Z
+name: Jake
+";
+
+            var sut = new DeserializerBuilder()
+                .WithNamingConvention(CamelCaseNamingConvention.Instance)
+                .WithDuplicateKeyChecking()
+                .Build();
+
+            Action act = () => sut.Deserialize<Person>(yaml);
+            act.ShouldThrow<YamlException>("Because there are duplicate name keys");
+        }
+
+        [Fact]
+        public void DeserializeWithoutDuplicateKeyChecking_YamlWithDuplicateKeys_DoesNotThrowYamlException()
+        {
+            var yaml = @"
+name: Jack
+momentOfBirth: 1983-04-21T20:21:03.0041599Z
+name: Jake
+";
+
+            var sut = new DeserializerBuilder()
+                .WithNamingConvention(CamelCaseNamingConvention.Instance)
+                .Build();
+
+            Action act = () => sut.Deserialize<Person>(yaml);
+            act.ShouldNotThrow<YamlException>("Because duplicate key checking is not enabled");
+        }
+
         public class Test
         {
             public string Value { get; set; }

YamlDotNet/Serialization/DeserializerBuilder.cs

@@ -47,6 +47,7 @@ public sealed class DeserializerBuilder : BuilderSkeleton<DeserializerBuilder>
         private readonly Dictionary<TagName, Type> tagMappings;
         private readonly Dictionary<Type, Type> typeMappings;
         private bool ignoreUnmatched;
+        private bool duplicateKeyChecking;
         private bool attemptUnknownTypeDeserialization;
 
         /// <summary>
@@ -85,7 +86,7 @@ public DeserializerBuilder()
                 { typeof(DictionaryNodeDeserializer), _ => new DictionaryNodeDeserializer(objectFactory.Value) },
                 { typeof(CollectionNodeDeserializer), _ => new CollectionNodeDeserializer(objectFactory.Value) },
                 { typeof(EnumerableNodeDeserializer), _ => new EnumerableNodeDeserializer() },
-                { typeof(ObjectNodeDeserializer), _ => new ObjectNodeDeserializer(objectFactory.Value, BuildTypeInspector(), ignoreUnmatched) }
+                { typeof(ObjectNodeDeserializer), _ => new ObjectNodeDeserializer(objectFactory.Value, BuildTypeInspector(), ignoreUnmatched, duplicateKeyChecking) }
             };
 
             nodeTypeResolverFactories = new LazyComponentRegistrationList<Nothing, INodeTypeResolver>
@@ -388,6 +389,16 @@ public DeserializerBuilder IgnoreUnmatchedProperties()
             return this;
         }
 
+        /// <summary>
+        /// Instructs the deserializer to check for duplicate keys and throw an exception if duplicate keys are found.
+        /// </summary>
+        /// <returns></returns>
+        public DeserializerBuilder WithDuplicateKeyChecking()
+        {
+            duplicateKeyChecking = true;
+            return this;
+        }
+
         /// <summary>
         /// Creates a new <see cref="Deserializer" /> according to the current configuration.
         /// </summary>

YamlDotNet/Serialization/NodeDeserializers/ObjectNodeDeserializer.cs

@@ -20,6 +20,7 @@
 // SOFTWARE.
 
 using System;
+using System.Collections.Generic;
 using System.Runtime.Serialization;
 using YamlDotNet.Core;
 using YamlDotNet.Core.Events;
@@ -32,12 +33,14 @@ public sealed class ObjectNodeDeserializer : INodeDeserializer
         private readonly IObjectFactory objectFactory;
         private readonly ITypeInspector typeDescriptor;
         private readonly bool ignoreUnmatched;
+        private readonly bool duplicateKeyChecking;
 
-        public ObjectNodeDeserializer(IObjectFactory objectFactory, ITypeInspector typeDescriptor, bool ignoreUnmatched)
+        public ObjectNodeDeserializer(IObjectFactory objectFactory, ITypeInspector typeDescriptor, bool ignoreUnmatched, bool duplicateKeyChecking)
         {
             this.objectFactory = objectFactory ?? throw new ArgumentNullException(nameof(objectFactory));
             this.typeDescriptor = typeDescriptor ?? throw new ArgumentNullException(nameof(typeDescriptor));
             this.ignoreUnmatched = ignoreUnmatched;
+            this.duplicateKeyChecking = duplicateKeyChecking;
         }
 
         bool INodeDeserializer.Deserialize(IParser parser, Type expectedType, Func<IParser, Type, object?> nestedObjectDeserializer, out object? value)
@@ -52,11 +55,17 @@ bool INodeDeserializer.Deserialize(IParser parser, Type expectedType, Func<IPars
             var implementationType = Nullable.GetUnderlyingType(expectedType) ?? expectedType;
 
             value = objectFactory.Create(implementationType);
+            var consumedProperties = new List<string>();
             while (!parser.TryConsume<MappingEnd>(out var _))
             {
                 var propertyName = parser.Consume<Scalar>();
+                if (duplicateKeyChecking && consumedProperties.Contains(propertyName.Value))
+                {
+                    throw new YamlException(propertyName.Start, propertyName.End, $"Encountered duplicate key {propertyName.Value}");
+                }
                 try
                 {
+                    consumedProperties.Add(propertyName.Value);
                     var property = typeDescriptor.GetProperty(implementationType, null, propertyName.Value, ignoreUnmatched);
                     if (property == null)
                     {