Xamarin.Android: item previously inserted in ArrayAdapter is not found again - xamarin

Question

I've inherited this Xamarin.Android app and it has a few issues.
A particular bug involves an ArrayAdapter<ProductListObject>, where ProductListObject is a common POCO that's shared between subprojects (i.e. Android, Windows Phone and iOS); it just has a couple of properties (e.g. an Id) and overrides the (.NET) Equals() method to achieve structural equality:
public class ProductListObject
{
public long Id { get; set; }
public override bool Equals(object obj)
{
if (!(obj is ProductListObject))
{
return false;
}
return Id == (obj as ProductListObject).Id;
}
}
The problem is that whenever I put an instance of this ProductListObject in an ArrayAdapter, I can't find it again, even if they have the same Id:
var p1 = new ProductListObject { Id = 1 };
var p2 = new ProductListObject { Id = 1 };
var areEqual = p1.Equals(p2); // returns True, as expected
var productAdapter = new ArrayAdapter<ProductListObject>(this, 0, new[] { p1 });
var position = productAdapter.GetPosition(p2); // returns -1 >:(
My question is: what do I have to do to make my POCO's work with Xamarin.Android types that rely on the Java equals() method, internally (like ArrayAdapter; which delegates to List.indexOf(Object))?
What I have tried:
verified that the corresponding Java version works as expected (it does)
overrode GetHashCode() (it doesn't matter, as I expected)
googled and checked the Xamarin documentation for information about implementing Equals() (I found nothing particularly relevant)
Thanks,
Jan

Answer 1

I did as Matt R suggested and created a proxy that inherits from Java.Lang.Object and delegates to the actual .NET object:
public class JavaObject<TValue> : Java.Lang.Object
{
public readonly TValue Value;
internal JavaObject(TValue value)
{
Value = value;
}
public override bool Equals(Java.Lang.Object that)
{
if (!(that is JavaObject<TValue>))
{
return false;
}
return Value.Equals((that as JavaObject<TValue>).Value);
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
}
This doesn't tie my platform-agnostic POCO's to the Android implementation, plus it doesn't force me to lock into some rigid inheritance tree, which is always a plus.
Application is straightforward:
var p1 = new JavaObject<ProductListObject>(new ProductListObject { Id = 1 });
var p2 = new JavaObject<ProductListObject>(new ProductListObject { Id = 1 });
var areEqual = p1.Equals(p2); // returns True, as expected
var productAdapter = new ArrayAdapter<JavaObject<ProductListObject>>(this, 0, new[] { p1 });
var position = productAdapter.GetPosition(p2); // returns 0!

Answer 2

It looks like .NET objects get wrapped within a Java.Lang.Object when used inside a Android.Widget.ArrayAdapter. Therefore, the comparison method that's used in the productAdapter.GetPosition(...) call is actually the java Equals(Java.Lang.Object o) method for the wrapping Java.Lang.Object.
To make a ProductListObject resolve to the same index when two objects have the same Id, make ProductListObject derive from Java.Lang.Object, override the Equals(Java.Lang.Object) and forward it to the .NET Equals(System.Object) method:
public class ProductListObject : Java.Lang.Object
{
public long Id { get; set; }
public override bool Equals(object obj) // Inherited from System.Object.
{
if (!(obj is ProductListObject))
{
return false;
}
return Id == (obj as ProductListObject).Id;
}
public override bool Equals (Java.Lang.Object o) // Inherited from Java.Lang.Object.
{
return this.Equals (o as System.Object);
}
}
If you can't inherit ProductListObject from Java.Lang.Object, another option is to implement your own proxy class:
public class ProductListObject
{
public long Id { get; set; }
public override bool Equals(System.Object obj)
{
if (!(obj is ProductListObject))
{
return false;
}
return Id == (obj as ProductListObject).Id;
}
}
public class JavaProxy: Java.Lang.Object
{
public Object Object { get; private set; }
public JavaProxy(System.Object o)
{
Object = o;
}
public override bool Equals (Java.Lang.Object o)
{
var proxy = o as JavaProxy;
if (o != null) {
return Object.Equals (proxy.Object);
}
return base.Equals (o);
}
}
// ...
var productAdapter = new ArrayAdapter<JavaProxy>(this, 0, new[] { new JavaProxy(p1) });
var position = productAdapter.GetPosition(new JavaProxy(p2));
It's not as clean as the first approach but it also works.