[release/10.0] Fix crash on calling COM interface method on a class where a derived class overriding it was created first

[elinor-fung]

See #127512

When a COM interface is implemented by a class with a virtual method and a derived class overrides it, they share a ComMethodTable. We then use GetSlotForVirtual to resolve to the correct target for a method. With #101580, the slot for the base class is lazily populated. If the derived class is used first it creates a ComMethodTable and lays it out, which makes sure the slots for that class' method table are restored. When base class is then used, it shares the same ComMethodTable which is already laid out, so it does not explicitly restore its slots, leaving the slot for the base class method null.

This change makes laying out a ComMethodTable also restore parent vtable slots. I did this directly against release/10.0, as how we do this dispatch is quite different in .NET 11. It doesn't crash anymore, but we do still have a correctness issue there.

cc @dotnet/appmodel @AaronRobinsonMSFT

Example setup:

[ComVisible(true)]
public interface IFoo
{
    void DoWork();
}
 
[ComVisible(true), ComDefaultInterface(typeof(IFoo))]
public class Base : IFoo
{
    public virtual void DoWork() { }
}
 
[ComVisible(true), ComDefaultInterface(typeof(IFoo))]
public class Derived : Base
{
    public override void DoWork() { }
}

If Derived is created first, calling Base.DoWork crashes.

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[Copilot]

Pull request overview

Backport to release/10.0 of a fix for a regression introduced by lazy temporary-entrypoint allocation (#101580). When a COM-visible base class and a derived class that overrides one of its virtual methods share a ComMethodTable, creating the derived class first laid out the shared ComMethodTable without ever forcing the base class's vtable slot to be restored. A subsequent COM call on the base instance then dispatched through a NULL slot and crashed. The fix walks parent MethodTables while laying out the interface method table and calls GetRestoredSlot so the base-class slot is materialized.

Changes:

• In ComMethodTable::LayOutInterfaceMethodTable, for each virtual class method bound to an interface slot, walk up the parent chain and call GetRestoredSlot(slot) for every parent that still defines that virtual.
• Adds a new COM interop test (VirtualMethodOverrideTest) with both derived-first and base-first call orders to guard against the regression.

Show a summary per file

File	Description
src/coreclr/vm/comcallablewrapper.cpp	Restore parent vtable slots when laying out a shared ComMethodTable to prevent NULL COM dispatch slots.
src/tests/Interop/COM/VirtualMethodOverride/VirtualMethodOverrideTest.cs	New xUnit tests covering both ordering scenarios for COM-visible base/derived classes sharing an interface.
src/tests/Interop/COM/VirtualMethodOverride/VirtualMethodOverrideTest.csproj	Test project wiring (process-isolated, NativeAOT-incompatible).

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0