From 66253b7cdd9909c907bdf270d4214ab4a4bc737a Mon Sep 17 00:00:00 2001
From: Khanh Le <nkgotcode@users.noreply.github.com>
Date: Sat, 6 Jun 2026 09:05:19 -0500
Subject: [PATCH] Fix runtime type member writes for CLR wrappers

---
 .../Runtime/InteropExplicitTypeTests.cs       | 27 +++++++++++++++++++
 Jint/Runtime/Interop/ObjectWrapper.cs         | 11 +++++---
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/Jint.Tests/Runtime/InteropExplicitTypeTests.cs b/Jint.Tests/Runtime/InteropExplicitTypeTests.cs
index f202249511..18beb51f07 100644
--- a/Jint.Tests/Runtime/InteropExplicitTypeTests.cs
+++ b/Jint.Tests/Runtime/InteropExplicitTypeTests.cs
@@ -22,6 +22,16 @@ public class CI1 : Super, I1
         string I1.Name { get; } = "CI1 as I1";
     }
 
+    public class BaseValue
+    {
+        public string BaseOnly { get; } = "base";
+    }
+
+    public class DerivedValue : BaseValue
+    {
+        public int DerivedOnlyProperty { get; set; }
+    }
+
     public class Indexer<T>
     {
         private readonly T t;
@@ -67,6 +77,11 @@ public InterfaceHolder()
 
     }
 
+    public class BaseValueHolder
+    {
+        public BaseValue Value { get; } = new DerivedValue();
+    }
+
     private readonly Engine _engine;
     private readonly InterfaceHolder holder;
 
@@ -145,6 +160,18 @@ public void SuperClassFromIndexer()
         Assert.Equal(holder.IndexerSuper[0].Name, _engine.Evaluate("holder.IndexerSuper[0].Name"));
     }
 
+    [Fact]
+    public void DerivedRuntimePropertyFromBaseDeclaredProperty()
+    {
+        var engine = new Engine(options => options.Interop.ThrowOnUnresolvedMember = true);
+        var holder = new BaseValueHolder();
+        engine.SetValue("holder", holder);
+
+        Assert.Equal("base", engine.Evaluate("holder.Value.BaseOnly"));
+        engine.Execute("const obj = holder.Value; obj.DerivedOnlyProperty = 123;");
+        Assert.Equal(123, ((DerivedValue) holder.Value).DerivedOnlyProperty);
+    }
+
     public struct NullabeStruct : I1
     {
         public NullabeStruct()
diff --git a/Jint/Runtime/Interop/ObjectWrapper.cs b/Jint/Runtime/Interop/ObjectWrapper.cs
index 585376403d..1c0c169ce8 100644
--- a/Jint/Runtime/Interop/ObjectWrapper.cs
+++ b/Jint/Runtime/Interop/ObjectWrapper.cs
@@ -215,7 +215,7 @@ public override bool Set(JsValue property, JsValue value, JsValue receiver)
             if (_properties is null || !_properties.ContainsKey(member))
             {
                 // can try utilize fast path
-                var accessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, ClrType, member, mustBeReadable: false, mustBeWritable: true);
+                var accessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, ClrType, member, mustBeReadable: false, mustBeWritable: true, throwOnError: false);
                 var actualType = Target.GetType();
                 if (ClrType != actualType)
                 {
@@ -224,7 +224,7 @@ public override bool Set(JsValue property, JsValue value, JsValue receiver)
                     // that should take precedence over the indexer
                     if (accessor is IndexerAccessor)
                     {
-                        var runtimeAccessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, actualType, member, mustBeReadable: false, mustBeWritable: true);
+                        var runtimeAccessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, actualType, member, mustBeReadable: false, mustBeWritable: true, throwOnError: false);
                         if (runtimeAccessor is not IndexerAccessor && runtimeAccessor != ConstantValueAccessor.NullAccessor)
                         {
                             accessor = runtimeAccessor;
@@ -232,12 +232,17 @@ public override bool Set(JsValue property, JsValue value, JsValue receiver)
                     }
                     else if (ReferenceEquals(accessor, ConstantValueAccessor.NullAccessor))
                     {
-                        accessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, actualType, member, mustBeReadable: false, mustBeWritable: true);
+                        accessor = _engine.Options.Interop.TypeResolver.GetAccessor(_engine, actualType, member, mustBeReadable: false, mustBeWritable: true, throwOnError: false);
                     }
                 }
 
                 if (ReferenceEquals(accessor, ConstantValueAccessor.NullAccessor))
                 {
+                    if (_engine.Options.Interop.ThrowOnUnresolvedMember)
+                    {
+                        throw new MissingMemberException($"Cannot access property '{member}' on type '{ClrType.FullName}");
+                    }
+
                     // there's no such property, but we can allow extending by calling base
                     // which will add properties, this allows for example JS class to extend a CLR type
                     return base.Set(property, value, receiver);