deviceDisplay.MainDisplayInfo = 'deviceDisplay.MainDisplayInfo' threw an exception of type 'Xamarin.Essentials.NotImplementedInReferenceAssemblyException'

[LeoJHarris]

Hi there,

First of all thanks for making this nuget available. Im just having an issue though with the IDeviceDisplay and calling the MainDisplayInfo. In my app in the App.cs class I am checking the device main display info and the exception being raised is:

deviceDisplay.MainDisplayInfo = 'deviceDisplay.MainDisplayInfo' threw an exception of type 'Xamarin.Essentials.NotImplementedInReferenceAssemblyException'

Would this be because I'm running unit tests without on an actual device and there is simply no implementation of it therefore?

I actually Registered the service prior to calling it using FreshMVVM i.e.

 private void setupIOC()
        {
...
            FreshIOC.Container.Register<IDeviceDisplay, DeviceDisplayImplementation>();
        }

Then I resolve it later in app.cs class but exception occuring here:

            IDeviceDisplay deviceDisplay = FreshIOC.Container.Resolve<IDeviceDisplay>();

            // Get Metrics
            DisplayInfo mainDisplayInfo = deviceDisplay.MainDisplayInfo;

Anyway to resolve this so it returns DisplayInfo?

[LeoJHarris]

Im guessing its better to avoid the call to MainDisplayInfo with a condition check i.e if not supported like following:

if (DeviceInfo.Platform != DevicePlatform.Unknown)
            {
                   Safe to call deviceDisplay.MainDisplayInfo;
             }

[rdavisau]

Howdy - yes, the error is saying that the display API has no implementation in the reference (shared/cross-platform) assembly, because the implementations are provided for each specific platform (iOS/Android, etc.). In a 'headless' scenario like you are describing, you have no platform implementation so you end up with this.

Im guessing its better to avoid the call to MainDisplayInfo with a condition check i.e if not supported like following:

That should work; another test-friendly approach would be to register a mock implementation instead, like:

public class UnitTestDeviceDisplay : IDeviceDisplay
{
	public bool KeepScreenOn { get; set; }

	public DisplayInfo MainDisplayInfo => new DisplayInfo();
		
	public event EventHandler<DisplayInfoChangedEventArgs> MainDisplayInfoChanged;
}

Or you could use a mocking library, particularly if you need values on MainDisplayInfo to be sensible (in the above width/height etc. will be zero).

[LeoJHarris]

Avoiding the call works for now, the only reason MainInfo needs to be called is because I used a different ResourceDictionary with styles dependant on that device size. However in a unit test scenario this is fairly trivial. If I do need values Ill mock it with NSubstitute with I use for mocking. Appreciate the help. Thanks