Serialize types implementing only IReadOnlyDictionary<,> as mappings

YamlDotNet.Test/Serialization/GenericTestReadOnlyDictionary.cs

@@ -0,0 +1,85 @@
+// This file is part of YamlDotNet - A .NET library for YAML.
+// Copyright (c) Antoine Aubry and contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of
+// this software and associated documentation files (the "Software"), to deal in
+// the Software without restriction, including without limitation the rights to
+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+// of the Software, and to permit persons to whom the Software is furnished to do
+// so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+using System.Collections;
+using System.Collections.Generic;
+
+namespace YamlDotNet.Test.Serialization
+{
+    /// <summary>
+    /// Test Dictionary that implements <see cref="IReadOnlyDictionary{TKey, TValue}" />, but not
+    /// <see cref="IDictionary{TKey, TValue}"/> nor <see cref="IDictionary"/>.
+    /// </summary>
+    public class GenericTestReadOnlyDictionary<TKey, TValue> : IReadOnlyDictionary<TKey, TValue>
+    {
+        private readonly Dictionary<TKey, TValue> dictionary;
+
+        public GenericTestReadOnlyDictionary()
+        {
+            dictionary = new Dictionary<TKey, TValue>();
+        }
+
+        public void Add(TKey key, TValue value)
+        {
+            dictionary.Add(key, value);
+        }
+
+        public TValue this[TKey key]
+        {
+            get { return dictionary[key]; }
+        }
+
+        public IEnumerable<TKey> Keys
+        {
+            get { return dictionary.Keys; }
+        }
+
+        public IEnumerable<TValue> Values
+        {
+            get { return dictionary.Values; }
+        }
+
+        public int Count
+        {
+            get { return dictionary.Count; }
+        }
+
+        public bool ContainsKey(TKey key)
+        {
+            return dictionary.ContainsKey(key);
+        }
+
+        public bool TryGetValue(TKey key, out TValue value)
+        {
+            return dictionary.TryGetValue(key, out value);
+        }
+
+        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
+        {
+            return dictionary.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return dictionary.GetEnumerator();
+        }
+    }
+}

YamlDotNet.Test/Serialization/SerializationTests.cs

@@ -1577,6 +1577,29 @@ public void SerializeGenericDictionaryPropertyAndDoNotApplyNamingConvention()
             writer.ToString().Should().Contain("new_key_here: new_value");
         }
 
+        [Fact]
+        public void SerializeGenericReadOnlyDictionary()
+        {
+            // A type that implements only IReadOnlyDictionary<,> must serialize as a
+            // mapping, the same as a regular dictionary, rather than as a sequence of
+            // key/value pairs. See https://github.com/aaubry/YamlDotNet/issues/606.
+            var readOnlyDictionary = new GenericTestReadOnlyDictionary<string, string>();
+            readOnlyDictionary.Add("key1", "value1");
+            readOnlyDictionary.Add("key2", "value2");
+
+            var serializer = new SerializerBuilder().Build();
+
+            var actual = serializer.Serialize(readOnlyDictionary);
+            var expected = serializer.Serialize(
+                new Dictionary<string, string>
+                {
+                    ["key1"] = "value1",
+                    ["key2"] = "value2",
+                });
+
+            actual.Should().Be(expected);
+        }
+
         [Theory, MemberData(nameof(SpecialFloats))]
         public void SpecialFloatsAreHandledCorrectly(FloatTestCase testCase)
         {

YamlDotNet/Helpers/GenericReadOnlyDictionaryToNonGenericAdapter.cs

@@ -0,0 +1,168 @@
+// This file is part of YamlDotNet - A .NET library for YAML.
+// Copyright (c) Antoine Aubry and contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of
+// this software and associated documentation files (the "Software"), to deal in
+// the Software without restriction, including without limitation the rights to
+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+// of the Software, and to permit persons to whom the Software is furnished to do
+// so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace YamlDotNet.Helpers
+{
+    /// <summary>
+    /// Adapts an <see cref="System.Collections.Generic.IReadOnlyDictionary{TKey, TValue}" /> to <see cref="IDictionary" />
+    /// so that types which implement only the read-only interface are still traversed as mappings.
+    /// Only enumeration is supported, which is all that object graph traversal requires.
+    /// </summary>
+    internal sealed class GenericReadOnlyDictionaryToNonGenericAdapter<TKey, TValue> : IDictionary
+        where TKey : notnull
+    {
+        private readonly IReadOnlyDictionary<TKey, TValue> genericDictionary;
+
+        public GenericReadOnlyDictionaryToNonGenericAdapter(IReadOnlyDictionary<TKey, TValue> genericDictionary)
+        {
+            this.genericDictionary = genericDictionary ?? throw new ArgumentNullException(nameof(genericDictionary));
+        }
+
+        public void Add(object key, object? value)
+        {
+            throw new NotSupportedException();
+        }
+
+        public void Clear()
+        {
+            throw new NotSupportedException();
+        }
+
+        public bool Contains(object key)
+        {
+            throw new NotSupportedException();
+        }
+
+        public IDictionaryEnumerator GetEnumerator()
+        {
+            return new DictionaryEnumerator(genericDictionary.GetEnumerator());
+        }
+
+        public bool IsFixedSize
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public bool IsReadOnly
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public ICollection Keys
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public void Remove(object key)
+        {
+            throw new NotSupportedException();
+        }
+
+        public ICollection Values
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public object? this[object key]
+        {
+            get
+            {
+                throw new NotSupportedException();
+            }
+            set
+            {
+                throw new NotSupportedException();
+            }
+        }
+
+        public void CopyTo(Array array, int index)
+        {
+            throw new NotSupportedException();
+        }
+
+        public int Count
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public bool IsSynchronized
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public object SyncRoot
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        private class DictionaryEnumerator : IDictionaryEnumerator
+        {
+            private readonly IEnumerator<KeyValuePair<TKey, TValue>> enumerator;
+
+            public DictionaryEnumerator(IEnumerator<KeyValuePair<TKey, TValue>> enumerator)
+            {
+                this.enumerator = enumerator;
+            }
+
+            public DictionaryEntry Entry
+            {
+                get
+                {
+                    return new DictionaryEntry(Key, Value);
+                }
+            }
+
+            public object Key
+            {
+                get { return enumerator.Current.Key!; }
+            }
+
+            public object? Value
+            {
+                get { return enumerator.Current.Value; }
+            }
+
+            public object Current
+            {
+                get { return Entry; }
+            }
+
+            public bool MoveNext()
+            {
+                return enumerator.MoveNext();
+            }
+
+            public void Reset()
+            {
+                enumerator.Reset();
+            }
+        }
+    }
+}

YamlDotNet/Serialization/ObjectFactories/ObjectFactoryBase.cs

@@ -63,6 +63,16 @@ public virtual bool GetDictionary(IObjectDescriptor descriptor, out IDictionary?
                 dictionary = adaptedDictionary as IDictionary;
                 return true;
             }
+
+            var genericReadOnlyDictionaryType = descriptor.Type.GetImplementationOfOpenGenericInterface(typeof(IReadOnlyDictionary<,>));
+            if (genericReadOnlyDictionaryType != null)
+            {
+                genericArguments = genericReadOnlyDictionaryType.GetGenericArguments();
+                var adaptedDictionary = Activator.CreateInstance(typeof(GenericReadOnlyDictionaryToNonGenericAdapter<,>).MakeGenericType(genericArguments), descriptor.Value)!;
+                dictionary = adaptedDictionary as IDictionary;
+                return true;
+            }
+
             genericArguments = null;
             dictionary = null;
             return false;