MissingMethodException in iOS app while using Xamarin's FileHelper Implementation - sqlite

Question

I recently followed Xamarin's SQLite tutorial to install SQLite-net PCL. Everything works perfectly on the simulator in Debug mode but I'm getting crashes on startup in Release mode.
The exception is as follows:
exception:
System.MissingMethodException
message:
Default constructor not found for type MyApp.iOS.FileHelper
stack trace:
at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain
(int,string[],intptr,intptr) at UIKit.UIApplication.Main
(System.String[] args, System.IntPtr principal, System.IntPtr
delegate) [0x00005] in
/Users/builder/data/lanes/3969/7beaef43/source/xamarin-macios/src/UIKit/UIApplication.cs:79
at UIKit.UIApplication.Main (System.String[] args, System.String
principalClassName, System.String delegateClassName) [0x00038] in
/Users/builder/data/lanes/3969/7beaef43/source/xamarin-macios/src/UIKit/UIApplication.cs:63
at MyApp.iOS.Application.Main (System.String[] args) [0x00008] in
/Users/{FilePath}/iOS/Main.cs:13
What I've found
So this MyApp.iOS.FileHelper is Xamarin's code that fetches the documents directory. The implementation goes like this:
In the Forms application we just have a contract:
public interface IFileHelper
{
string GetLocalFilePath(string filename);
}
In the MyApp.iOS project we define a dependency:
[assembly: Dependency(typeof(FileHelper))]
namespace MyApp.iOS
{
public class FileHelper : IFileHelper
{
public FileHelper() { }
public string GetLocalFilePath(string filename)
{
string docFolder = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
string libFolder = Path.Combine(docFolder, "..", "Library", "Databases");
if (!Directory.Exists(libFolder))
{
Directory.CreateDirectory(libFolder);
}
return Path.Combine(libFolder, filename);
}
}
}
Here's how I am using it:
DependencyService.Get<IFileHelper>().GetLocalFilePath("MyAppDatabase.db3")
The application works as expected in Debug mode. So either the Dependency Service is behaving differently in Release or the Documents Directory is different in Release.
My Question
How can avoid this exception in release mode?
Dependency Info:
Xamarin Forms 2.3.3.193
Sqlite-net-pcl 1.2.1
update:
What I've tried:
Added a default constructor, yields no change
Tried different linker possibilities, yields no change

Answer 1

From Introduction to DependencyService
Note that every implementation must have a default (parameterless)
constructor in order for DependencyService to be able to instantiate
it. Parameterless constructors cannot be defined by the interface.
You need to add an empty constructor to your FileHelper class.
public FileHelper() {
}

Answer 2

Please note: I tried voting to close this question as a duplicate. It clearly
failed to be closed as such.
Based on comments point to this post from SushiHangover, this is what resolved my issue:
I needed to add decorate my FileHelper class in my iOS project with:
[Preserve(AllMembers = true)]
again, please go check out his original post here:
https://stackoverflow.com/a/41932246/1664443