ResourceProvider Exception in Xamarin.Forms 3.0 due to Signature Change

[LanceMcCarthy]

Description

Due to the change made in #1976 for Xamarin.Forms 3.0, ResourceDictionaries that were compiled with XF 2.4 fail to load throwing the following exception:

Can't resolve the reference 'System.Func`2<System.String,System.String> Xamarin.Forms.Internals.ResourceLoader::get_ResourceProvider()', referenced from the method 'System.Void TheExternalResourceDictionary::InitializeComponent()' in 'Xamarin.Forms.Core

In particular, the ResourceDictionary in this case contains the styles for custom Xamarin.Forms controls that have a minimum support Xamarin.Forms version of 2.4

Steps to Reproduce

Download the simple repro using the link at the bottom of this post. Let me walk you through the two solutions:

1. CustomControls - A simple class library with the ResourceDictionary
2. ResLoaderRepro - The Xamarin.Forms runtime app that present the problem

1- CustomControls.sln (aka CoolGrid)

This is very simple class library that contains one class, CoolGrid, that inherits from Xamarin.Forms.Grid.

public class CoolGrid : Xamarin.Forms.Grid
    {
        public CoolGrid()
        {
            this.Resources = new CoolGridTheme();
        }
    }

The ResourceDictionary, "CoolGridTheme.xaml", is also very simple and contains a couple OnPlatform for height and a Style to set CoolGrid's BackgroundColor

<OnPlatform x:TypeArguments="x:Double" x:Key="Height" Default="32">
    <On Platform="Android" Value="45"/>
    <On Platform="iOS" Value="36"/>
    <On Platform="UWP" Value="32"/>
</OnPlatform>
<OnPlatform x:TypeArguments="x:Double" x:Key="MinimumHeight" Default="33">
    <On Platform="Android" Value="28"/>
    <On Platform="iOS" Value="28"/>
    <On Platform="UWP" Value="33"/>
</OnPlatform>
    
<Style x:Key="BaseCoolGridStyle" TargetType="customControls:CoolGrid">
    <Setter Property="HeightRequest" Value="{StaticResource Height}"/>
    <Setter Property="MinimumHeightRequest" Value="{StaticResource MinimumHeight}"/>
    <Setter Property="BackgroundColor" Value="DarkGray"/>
</Style>

The compiled result was copied to the Libs folder in the 2nd solution's folder.

2 - ResLoaderRepro.sln

This solution contains the Xamarin.Forms 3.0 projects (created using the File > New > Xamarin.Forms app > Blank App template and only kept Android to keep it small).

I updated the XF NuGet page to the latest public release and added the reference to the CustomControls assembly in the Libs folder.

Finally, on MainPage, I add a single instance of CoolGrid:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:customControls="clr-namespace:CustomControls;assembly=CustomControls"
             x:Class="ResLoaderRepro.MainPage">
    
    <StackLayout>
        <customControls:CoolGrid x:Name="MyCoolGrid" HeightRequest="100" WidthRequest="100"/>
    </StackLayout>
</ContentPage>

Here's a screenshot to better explain the simplicity of the setup:

Expected Behavior

The CoolGrid control will render.

Actual Behavior

An exception is thrown (see scrrenshot 4 below

System.MissingMethodException: System.Func`2<string, string> Xamarin.Forms.Internals.ResourceLoader.get_ResourceProvider()

Here's a screenshot of the break:

Basic Information

• Version with issue:
• Last known good version: v2.5.1.527436
• IDE: VS2017 15.7.2
• Platform Target Frameworks: All
• Android Support Library Version: 23.3+
• Nuget Packages: XF 3.0+

Screenshots

Screenshots inline with the explanation above.

Reproduction Link

Download the reproducible from here (3.1 MB)

[PureWeen]

xamarin/xamarin-macios#3639

The solution here
https://bugzilla.xamarin.com/show_bug.cgi?id=60949
Was just to downgrade but that doesn't seem to work in your case

I also converted your project to netstandard since it was .net 4.5 just to see and same issue

[Szymaniuk]

Any chance for quick fix? It's blocking me from updating my app to any Xamarin.Forms greater than 2.5.1.527436...

[StephaneDelcroix]

I'm literally investigating this right now, but still can't tell if we'll be able to deliver a fix, nor can't I give you a time frame.

The workaround is to make sure all your dependencies are compiled against latest XF 3.0.0

[LanceMcCarthy]

I can confirm #2880 fixes the problem in my tests (attached repro and commercial 3rd party libraries).

Repro Screenshot (using new Xamarin.Forms package containing fix):

[StephaneDelcroix]

@LanceMcCarthy could you also confirm it works on iOS ? thanks

[LanceMcCarthy]

@StephaneDelcroix confirmed working on iOS 11.4, iPhone 8

[avintoptaros]

Hi @StephaneDelcroix , do you know when the next release will be with this fix? Thank you.

[stephanosio]

@avintoptaros It looks like the fix is already included in the release-3.0.0-sr3 and beta-3.1.0-pre3.

[avintoptaros]

@stephanosio I see that on the release page there is no release-3.0.0-sr3 version (https://developer.xamarin.com/releases/xamarin-forms/xamarin-forms-3.0/)

[stephanosio]

@avintoptaros Try this: https://github.com/xamarin/Xamarin.Forms/releases