How do I properly overload the C# Item property in an interface for COM?

I'm trying to write a class in C# that supports the Item operators in VB6 (obj("Name") and obj!Name) through COM. I'm using a public class with a corresponding interface.

I have successfully overloaded the square bracket operators in C# and I can call the getter function with the proper operators from VB6, but I'm having trouble with calling the setter function. The error I usually get is "Object doesn't support this property or method".

Here's where it get's tricky:

When I don't define the interface and use the default COM object that is generated from my class, I can successfully use both the getter and setter functions, along with all of my other public functions in the class. However, there is a serious performance degradation in the getter function (about 5x) when I do this, and that is something that I can't live with for this project.

I've looked into early/late binding, and I'm guessing this has something to do with it, but I don't know enough to figure out what is actually going on.

Can anyone explain this for me?

Here's my C# class and interface:

public interface IExampleClass
{
    bool Property1 { get; }

    void Update();
    void SetValue(string name, dynamic value)
    
    dynamic this[string name] { get; set; }
}

[ClassInterface(ClassInterfaceType.None)]
public class ExampleClass : IExampleClass
{
    private CustomObject obj;

    public ExampleClass()
    {
        this.obj = new CustomObject(); // This has data when initialized
    }

    public bool Property1 => obj.Property1;

    public void Update() => obj.Update();
    public void SetValue(string name, dynamic value) => obj.SetValue(name, value);

    public dynamic this[string name]
    {
        get
        {
             dynamic item = obj.GetValue(name);
             // do something with item
             return item;
        }
        set { obj.SetValue(name, value); }
    }
}

And my VB6 test script:

Sub TestItemProperty()
    Dim obj As ExampleClass
    Set obj = New ExampleClass
    
    'Test getter function
    MsgBox obj!Name               'This works
    MsgBox obj("Name")            'This works

    'Test setting
    obj.SetValue "Name", "John"   'This works

    'Test setter function
    obj!Name = "John"             'This doesn't work
    obj("Name") = "John"          'This doesn't work
End Sub