[xaml] locate event handlers from base types

[jonathanpeppers]

Fixes #11467
Context: https://github.com/maiia-kuzmishyna/MAUI-ProtectedEventHandlers

Reviewing the customer sample, it appears to be a valid case. Consider a ChildButton defined in XAML:

<local:ParentButton
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:local="clr-namespace:foo"
    x:Class="foo.ChildButton">
</local:ParentButton>

Where ParentButton is also defined in XAML:

<Button
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="foo.ParentButton"
    Clicked="ParentButton_OnClicked">
</Button>

Where ParentButton_OnClicked is of course private:

private void ParentButton_OnClicked(object sender, EventArgs e) { }

Throws an exception at runtime with:

No method ParentButton_OnClicked with correct signature found on type foo.ChildButton

What was also odd, it appears the problem just "goes away" in Release mode, meaning it works under XamlC. This kind of points to a bug with non-compiled XAML.

It appears the problem was the code:

foreach (var mi in rootElement.GetType().GetRuntimeMethods())
{
    //...
    if (mi.Name == (string)value)
    {
        addMethod.Invoke(element, new[] { mi.CreateDelegate(eventInfo.EventHandlerType, mi.IsStatic ? null : rootElement) });
        return true;
    }
    //...
}

In this example, rootElement is of type ChildButton while the method is on type ParentButton. As mentioned on:

https://stackoverflow.com/a/2267299

You need to access Type.BaseType to find private methods on base types.

I changed this to instead:

• Iterate over rootElement.GetType() and its base types.

• Call Type.GetMethod() using the method name, passing the appropriate BindingFlags.

This appears to make the new test pass, solving the issue.

[jonathanpeppers]

I added my own event here, so I could assert MyEventSubscriberCount is 1.