Fix IReadOnlyDictionary being treated as array-like instead of dictionary

Jint.Tests/Runtime/InteropTests.cs

@@ -90,6 +90,25 @@ public void EngineShouldStringifyADictionaryOfStringAndObjectCorrectly()
         Assert.Equal("{\"foo\":5,\"bar\":\"A string\"}", result);
     }
 
+    [Fact]
+    public void ReadOnlyDictionaryShouldNotBeTreatedAsArrayLike()
+    {
+        var engine = new Engine();
+
+        var dictionary = new ReadOnlyDictionary(new Dictionary<string, object>
+        {
+            { "foo", 5 },
+            { "bar", "A string" }
+        });
+        engine.SetValue(nameof(dictionary), dictionary);
+
+        var result = engine.Evaluate($"JSON.stringify({nameof(dictionary)})").AsString();
+        Assert.Equal("{\"foo\":5,\"bar\":\"A string\"}", result);
+
+        var keys = engine.Evaluate($"Object.keys({nameof(dictionary)})").AsArray();
+        Assert.Equal((uint) 2, keys.Length);
+    }
+
     [Fact]
     public void EngineShouldRoundtripParsedJSONBackToStringCorrectly()
     {
@@ -3655,6 +3674,28 @@ public Dictionary<string, int> Metadata
         }
     }
 
+    /// <summary>
+    /// A custom IReadOnlyDictionary that does NOT implement IDictionary, to verify it's treated as dictionary-like, not array-like.
+    /// </summary>
+    private class ReadOnlyDictionary : IReadOnlyDictionary<string, object>
+    {
+        private readonly Dictionary<string, object> _dictionary;
+
+        public ReadOnlyDictionary(Dictionary<string, object> dictionary)
+        {
+            _dictionary = dictionary;
+        }
+
+        public IEnumerator<KeyValuePair<string, object>> GetEnumerator() => _dictionary.GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        public int Count => _dictionary.Count;
+        public bool ContainsKey(string key) => _dictionary.ContainsKey(key);
+        public bool TryGetValue(string key, out object value) => _dictionary.TryGetValue(key, out value!);
+        public object this[string key] => _dictionary[key];
+        public IEnumerable<string> Keys => _dictionary.Keys;
+        public IEnumerable<object> Values => _dictionary.Values;
+    }
+
     [Fact]
     public void CanSelectShadowedPropertiesBasedOnReadableAndWritable()
     {

Jint/Runtime/Interop/TypeDescriptor.cs

@@ -17,6 +17,7 @@ internal sealed class TypeDescriptor
     private static readonly PropertyInfo _listIndexer = typeof(IList).GetProperty("Item")!;
 
     private static readonly Type _genericDictionaryType = typeof(IDictionary<,>);
+    private static readonly Type _readOnlyGenericDictionaryType = typeof(IReadOnlyDictionary<,>);
     private static readonly Type _stringType = typeof(string);
 
     private readonly MethodInfo? _tryGetValueMethod;
@@ -176,21 +177,26 @@ private static void AnalyzeType(
             var genericTypeDefinition = type.GetGenericTypeDefinition();
 
             // check if object has any generic dictionary signature that accepts string as key
-            var b = genericTypeDefinition == _genericDictionaryType;
-            if (b && type.GenericTypeArguments[0] == _stringType)
+            var isGenericDictionary = genericTypeDefinition == _genericDictionaryType;
+            var isReadOnlyGenericDictionary = genericTypeDefinition == _readOnlyGenericDictionaryType;
+            if ((isGenericDictionary || isReadOnlyGenericDictionary) && type.GenericTypeArguments[0] == _stringType)
             {
-                tryGetValueMethod = type.GetMethod("TryGetValue");
-                removeMethod = type.GetMethod("Remove");
-                keysAccessor = type.GetProperty("Keys");
-                valueType = type.GenericTypeArguments[1];
+                tryGetValueMethod ??= type.GetMethod("TryGetValue");
+                keysAccessor ??= type.GetProperty("Keys");
+                valueType ??= type.GenericTypeArguments[1];
+
+                if (isGenericDictionary)
+                {
+                    removeMethod ??= type.GetMethod("Remove");
+                }
             }
 
             isCollection |= genericTypeDefinition == typeof(IReadOnlyCollection<>) || genericTypeDefinition == typeof(ICollection<>);
             if (genericTypeDefinition == typeof(IList<>))
             {
                 integerIndexer ??= type.GetProperty("Item");
             }
-            isDictionary |= genericTypeDefinition == _genericDictionaryType;
+            isDictionary |= isGenericDictionary || isReadOnlyGenericDictionary;
         }
     }