How to get DPI device to PCL in Xamarin. Forms? - Followup - forms

Question

Using Xamarin Forms Visual Studio 2019.
I need to get the DPI of the device.
This post has a solution:
How to get DPI device to PCL in Xamarin. Forms?
Implementing the solution gave problems:
When applying the answer above I get this error message:
The type or namespace name 'DependencyAttribute' could not be found
(are you missing a using directive or an assembly reference?)
In your android implementation, add a new class:
[assembly: Dependency(typeof(DisplayInfo))]
namespace .....
--I solved this compiler problem by replacing with:
using Xamarin.Essentials;
[assembly: Xamarin.Forms.Dependency(typeof(DisplayInfo))]
However when I compile and run the code, it falls over when I try consume this in the Xamarin Core Code:
int dpi = DependencyService.Get<IDisplayInfo>().GetDisplayDpi();
I get this runtime error on the above line:
System.NullReferenceException: Object reference not set to an instance
of an object
Any ideas? I would have added this question as a comment on the original, but as a new user do not have the points to do this... If someone could drop a comment on the original question's solution pointing it to this URL that would be helpful too.

Answer 1

So the answer to my own question:
The solution original offered said do this:
In your android implementation, add a new class:
[assembly: Dependency(typeof(DisplayInfo))]
namespace YourAppNamespace.Droid
{
public class DisplayInfo : IDisplayInfo
{
public int GetDisplayWidth()
{
(int)Android.App.Application.Context.Resources.DisplayMetrics.WidthPixels;
}
public int GetDisplayHeight()
{
(int)Android.App.Application.Context.Resources.DisplayMetrics.HeightPixels;
}
public int GetDisplayDpi()
{
(int)Android.App.Application.Context.Resources.DisplayMetrics.DensityDpi;
}
}
}
The problem I did not see anywhere in the Android code that initialised this class, and I was unsure how best to do this really.
So I followed guidance for implementing the interface directly in the Android MainActivity.cs file, and this worked.. The video I watched to help me on that is here:
https://www.youtube.com/watch?v=lgcnYDb6cRQ&t=19s