What does assignment to a bracketed expression mean in C#?

Question

I'm reading Avalonia source code and I came across this sentence:

return new MenuFlyoutPresenter
{
    [!ItemsControl.ItemsProperty] = this[!ItemsProperty],
    [!ItemsControl.ItemTemplateProperty] = this[!ItemTemplateProperty]
};

I've never seen a syntax like that. What does those bracket do if there is no indexed property or this[] accessor?, and why are they negated with the exclamation mark if the property they are referring to is not a bool?, maybe some kind of null-check?

The code itself is contained in the following cs file:

https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Controls/Flyouts/MenuFlyout.cs

I've tracked the code but I was unable to understand what that syntax does.

Answer 1

There are a couple of things going on here.

First, the syntax:

var menu = new MenuFlyoutPresenter
{
    [key] = value,
};

Is a collection initializer, and is shorthand for:

var menu = new MenuFlyoutPresenter();
menu[key] = value;

That indexer is defined here as:

public IBinding this[IndexerDescriptor binding]
{
    get { return new IndexerBinding(this, binding.Property!, binding.Mode); }
    set { this.Bind(binding.Property!, value); }
}

So the key there is an IndexerDescriptor, and the value is an IBinding.

So, what's going on with this thing?

!ItemsControl.ItemsProperty

We can see from your link that ItemsProperty is a DirectProperty<TOwner, TValue>, and that ultimately implements the ! operator here:

public static IndexerDescriptor operator !(AvaloniaProperty property)
{
    return new IndexerDescriptor
    {
        Priority = BindingPriority.LocalValue,
        Property = property,
    };
}

Avalonia seems to like overloading operators such as ! and ~ to do things you might not expect (and would normally use a method for). In this case, they use ! on an AvaloniaProperty as a shorthand for accessing that property's binding.