Error trying to create master details page

Severity Code Description Project File Line Suppression State
Error CS0263 Partial declarations of 'MainPage' must not specify different base classes App3 \DEVELOPMENT\temp workspace\App3\App3\App3\MainPage.xaml.cs 10 Active
Error CS0029 Cannot implicitly convert type 'App3.MainPage' to 'Xamarin.Forms.Page' App3 \DEVELOPMENT\temp workspace\App3\App3\App3\App.xaml.cs 16 Active

I'm trying to create a hamburger menu using master details page, like in the GitHub sample...but when I changes MainPage base class from ContentPage to MasterDetailPage like this, I get the errors above

MY CODE

public partial class MainPage : MasterDetailPage
{
    public MainPage()
    {
        InitializeComponent();
    }
}

}

GitHub SAMPLE CODE

public partial class MainPage : MasterDetailPage
{
    public MainPage()
    {
        InitializeComponent();

        masterPage.ListView.ItemSelected += OnItemSelected;

        if (Device.RuntimePlatform == Device.UWP)
        {
            MasterBehavior = MasterBehavior.Popover;
        }
    }

    void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
    {
        var item = e.SelectedItem as MasterPageItem;
        if (item != null)
        {
            Detail = new NavigationPage((Page)Activator.CreateInstance(item.TargetType));
            masterPage.ListView.SelectedItem = null;
            IsPresented = false;
        }
    }
}

Honestly I don't understand how to do this....there is no step by step guide here at all how to create the navigation using master detail page. Just a bunch of code. Plus all the samples online are one or two years old - I don't even know if they are relevant for the lasts version.