DataTemplateSelector not found in merged resource dictionary

Issue

I added CustomDataTemplateSelector to a ResourceDictionary in a separated file called DataTemplates.xaml which I subsequently linked in App.xaml

When I run the app, I get XamlParseException saying: Type local:CustomDataTemplateSelector not found in xmlns clr-namespace:MyProject

On the other hand, when I put the content of DataTemplates.xaml to App.xaml, it works just fine.

App.xaml

<Application.Resources>
<ResourceDictionary>
    <ResourceDictionary Source="../DataTemplates.xaml"/>

DataTemplates.xaml

<?xml version="1.0" encoding="UTF-8"?>
<ResourceDictionary
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:MyProject">

<DataTemplate x:Key="FirstCell">
    <Label Text="Some text"/>
</DataTemplate>

<DataTemplate x:Key="SecondCell">
        <Label Text="Some other text"/>
</DataTemplate>

<local:CustomDataTemplateSelector
    x:Key="CustomDataTemplateSelector"
    SecondTemplate="{StaticResource SecondCell}"
    FirstTemplate="{StaticResource FirstCell}"/>

Solution

I've confirmed the problem. (To clarify for others - the parse error occurs when attempt to run the app. During App.InitializeComponent.) I had xmlns:local ... on both App.xaml and DataTemplates.xaml.

To work-around this, use an alternative syntax for merging dictionaries, ResourceDictionary.MergedDictionaries:

<Application xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:ios="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"
             xmlns:local="clr-namespace:XFSOAnswers"
             x:Class="XFSOAnswers.App">
    <Application.Resources>
        <ResourceDictionary>
            
            <Style TargetType="ContentPage" ApplyToDerivedTypes="True">
                <Setter Property="ios:Page.UseSafeArea" Value="True"/>
            </Style>

            <ResourceDictionary.MergedDictionaries>
                <local:DataTemplates />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

---

In case it matters, here is what I had for DataTemplates:

DataTemplates.xaml:

<?xml version="1.0" encoding="UTF-8"?>
<ResourceDictionary
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:XFSOAnswers">

    <DataTemplate x:Key="FirstCell">
        <Label Text="Some text"/>
    </DataTemplate>

    <DataTemplate x:Key="SecondCell">
        <Label Text="Some other text"/>
    </DataTemplate>

    <local:CustomDataTemplateSelector
    x:Key="CustomDataTemplateSelector"
    SecondTemplate="{StaticResource SecondCell}"
    FirstTemplate="{StaticResource FirstCell}"/>

    </ResourceDictionary>

DataTemplates.xaml.cs:

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace XFSOAnswers
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class DataTemplates : ResourceDictionary
    {
        public DataTemplates()
        {
        }
    }
}

(The original Source syntax should not need this .cs file. I don't know whether the syntax I used needs it or not.)

Answered By - ToolmakerSteve