Can't set properties on nested classes from Xaml

Question

The error that Xamarin gives back: Type Foo not found in xmlns clr-namespace:AmsterdamTheMapV3

The problem is that I am trying to define Foo but whatever I do it doenst work. I think that the problem is inside the xmls:local namespace. Maybe I make a stupid mistake inside my code.

This is a Xamarin.forms project for windows, android and apple.

Xaml file:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="AmsterdamTheMapV3.CityGuide"
             xmlns:local="clr-namespace:AmsterdamTheMapV3">
  <Label Text="{Binding MainText}" VerticalOptions="Center" HorizontalOptions="Center" />
  <ScrollView Orientation="Vertical" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
    <StackLayout Orientation="Vertical" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"  Spacing="0">

      <!-- Button1 -->
      <Image
          x:Name="CityGuideButton1"
          Source="shopping_gray.png"
          Aspect="Fill"
          HorizontalOptions="FillAndExpand"
                VerticalOptions ="FillAndExpand"
          local:Foo.Tag="button1">
        <Image.GestureRecognizers>
          <TapGestureRecognizer
            Tapped="Categorie_Onclick"
            NumberOfTapsRequired="1"/>
        </Image.GestureRecognizers>
      </Image>
    </StackLayout>
  </ScrollView>
</ContentPage>

Cs file:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;

namespace AmsterdamTheMapV3
{
    public partial class CityGuide : ContentPage
    {
        public CityGuide()
        {
            InitializeComponent();
        }

         public class Foo
        {
            public static readonly BindableProperty TagProperty = BindableProperty.Create("Tag", typeof(string), typeof(Foo), null);

            public static string GetTag(BindableObject bindable)
            {
                return (string)bindable.GetValue(TagProperty);
            }

            public static void SetTag(BindableObject bindable, string value)
            {
                bindable.SetValue(TagProperty, value);
            }
        }


        async void Categorie_Onclick(Object sender, EventArgs args)
        {
            Image cmd = sender as Image;
            string txt = Foo.GetTag(cmd);
            await Navigation.PushAsync(new CategoriePage(txt));
        }
    }
}

I hope that somewann can help me.
I can give more info if needed.
thank you in advance!

Answer 1

You can't reference nested classes from Xaml.

You can quote me on this, or google it, but this is an universal truth. The reason is that Xaml Parsers would not be able to differentiate an Attached (Bindable|Dependency)Property from a nested class access, based on the dot sign.

In your case, move your Foo class from within CityGuide directly to the namespace and it should work.

Your xmlns declaration xmlns:local="clr-namespace:AmsterdamTheMapV3" is correct without the assembly= part as you are referring to the current assembly.

More info on a similar answer:https://stackoverflow.com/a/14546917/1063783

Some might ask why not use the + sign for nested class (like it's done in Reflection). The problem is that an a name containing a + sign is not a valid xml-element name (https://www.xml.com/pub/a/2001/07/25/namingparts.html)