_In_ and Activated += ref new

Question

In learning Direct3D tutorial, I don't quite understand the following two statements:

    virtual void Initialize(
        _In_ CoreApplicationView^ applicationView
        )
    {
        applicationView->Activated +=
            ref new TypedEventHandler<CoreApplicationView^, IActivatedEventArgs^>(this, &Direct3DTutorialFrameworkView::OnActivated);
    }

Is the keyword _In_ similar to const of C++?

Why not write

applicationView->Activated = ref new TypedEventHandler...