Microsoft.Maui.IPropertyMapper - The type '' cannot be used as a type parameter... There is no implicit conversion

Question

I'm trying to get maps to work on .net MAUI and found this blog post (linked to from the official MAUI blog). https://www.cayas.de/blog/dotnet-maui-custom-map-handler I've downloaded the repo linked at the end of the blog but I can't get it to run.

In the blog post the MapHandler code looks like this:

public static IPropertyMapper<IMap, MapHandler> MapMapper = new PropertyMapper<IMap, MapHandler>(ViewMapper)
    {  };

But in the repo it looks like this:

public static IPropertyMapper<MapView, MapHandler> MapMapper = new PropertyMapper<MapView, MapHandler>(ViewMapper)
        { };

Neither of which work for me, as I get the error (at runtime) below.

The type 'MapControlDemo.Handlers.MapHandler' cannot be used as type parameter 'TViewHandler' in the generic type or method 'PropertyMapper<TVirtualView, TViewHandler>'. There is no implicit reference conversion from 'MapControlDemo.Handlers.MapHandler' to 'Microsoft.Maui.IElementHandler'

Confusingly there are another couple of errors that shouldn't be errors??

Any ideas? I contacted the author and he suggested it could be my maui version as it works for him, but I'm on the latest.

Answer 1

Perhaps the author was building on a Mac.

I've confirmed that an attempt to build on Windows results in a build error. VS Version 17.3.0 Preview 1.1
The error I got was

Error   CS0311  The type 'MapControlDemo.Handlers.MapHandler' cannot be used
 as type parameter 'TViewHandler'
 in the generic type or method 'IPropertyMapper<TVirtualView, TViewHandler>'.
 There is no implicit reference conversion from
 'MapControlDemo.Handlers.MapHandler' to
 'Microsoft.Maui.IElementHandler'.
    MapControlDemo (net6.0-android) C:\...\maui-maps\Handlers\MapHandler.cs 6

HOWEVER, I was able to RUN on Android - by selected an Android emulator as target (so using net6.0-android) and pressing F5 - therefore that error is misleading.

---

Given that this is only implemented on iOS and Android, I recommend editing .csproj to REMOVE Windows platform. I also removed MacCatalyst (maybe that would work but I was going for simplest test):
Change:

        <TargetFrameworks>net6.0-android;net6.0-ios;net6.0-maccatalyst</TargetFrameworks>
        <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net6.0-windows10.0.19041.0</TargetFrameworks>

To:

        <TargetFrameworks>net6.0-android;net6.0-ios</TargetFrameworks>

Then I was able to build.

NOTE: Without adding a Google Maps ID, when ran on Android, the result was an empty map.

tl;dr Easiest way to test is on a Mac, targetting iOS.