Xamarin SKPaymentQueue AddPayment throwing exception - xamarin.ios

Question

In my Xamarin app when I call this method
private void MakePayment (SKProduct product)
{
SKPayment payment = SKPayment.PaymentWithProduct (product);
SKPaymentQueue.DefaultQueue.AddPayment (payment);
}
I get this error:
Failed to marshal the Objective-C object 0x14607110 (type:
SKPaymentTransaction). Could not find an existing managed instance for
this object, nor was it possible to create a new managed instance
(because the type 'MonoTouch.StoreKit.SKPaymentTransaction[]' does not
have a constructor that takes one IntPtr argument).
I'm not sure if I have something configured wrong or there is an issue in my code or Xamarin.
Here's the code for the Observer
internal class CustomPaymentObserver : SKPaymentTransactionObserver
{
private InAppPurchase inAppPurchase;
public CustomPaymentObserver (InAppPurchase inAppPurchase)
{
this.inAppPurchase = inAppPurchase;
}
public override void UpdatedTransactions (SKPaymentQueue queue, SKPaymentTransaction[] transactions)
{
Console.WriteLine ("UpdatedTransactions");
foreach (SKPaymentTransaction transaction in transactions) {
switch (transaction.TransactionState) {
case SKPaymentTransactionState.Purchased:
inAppPurchase.CompleteTransaction (transaction);
break;
case SKPaymentTransactionState.Failed:
inAppPurchase.FailedTransaction (transaction);
break;
default:
break;
}
}
}
public override void PaymentQueueRestoreCompletedTransactionsFinished (SKPaymentQueue queue)
{
}
public override void RestoreCompletedTransactionsFailedWithError (SKPaymentQueue queue, NSError error)
{
}
}
Here is the full stack trace:
System.Exception: Failed to marshal the Objective-C object 0x17ecb680 (type: SKPaymentTransaction). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouch.StoreKit.SKPaymentTransaction[]' does not have a constructor that takes one IntPtr argument).
at MonoTouch.ObjCRuntime.Runtime.MissingCtor (IntPtr ptr, IntPtr klass, System.Type type, MissingCtorResolution resolution) [0x00046] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/.pp-Runtime.cs:352
at MonoTouch.ObjCRuntime.Runtime.ConstructNSObject[NSObject] (IntPtr ptr, System.Type type, MissingCtorResolution missingCtorResolution) [0x00000] in :0
at MonoTouch.ObjCRuntime.Runtime.GetNSObject (IntPtr ptr, System.Type target_type, MissingCtorResolution missingCtorResolution, System.Boolean& created) [0x00073] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/.pp-Runtime.cs:514
at MonoTouch.ObjCRuntime.Runtime.GetNSObjectWrapped (IntPtr ptr, IntPtr type_ptr, System.Boolean& created) [0x0000c] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/.pp-Runtime.cs:686
at at (wrapper native-to-managed) MonoTouch.ObjCRuntime.Runtime:GetNSObjectWrapped (intptr,intptr,int&)
at at (wrapper managed-to-native) MonoTouch.ObjCRuntime.Messaging:void_objc_msgSend_IntPtr (intptr,intptr,intptr)
at MonoTouch.StoreKit.SKPaymentQueue.AddPayment (MonoTouch.StoreKit.SKPayment payment) [0x00014] in /Developer/MonoTouch/Source/monotouch/src/StoreKit/.pp-SKPaymentQueue.g.cs:109
at IOS.Util.IAP.InAppPurchase.ReceivedResponse (MonoTouch.StoreKit.SKProductsRequest request, MonoTouch.StoreKit.SKProductsResponse response) [0x0001d] in /Users/aaron/Projects/budget-ease-xamarin/IOS/Util/IAP/InAppPurchase.cs:43
at at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/.pp-UIApplication.cs:38
at IOS.Application.Main (System.String[] args) [0x00008] in /Users/aaron/Projects/budget-ease-xamarin/IOS/Main.cs:16

Answer 1

What's happened here is that the C# instance of your CustomPaymentObserver got garbage collected, while it's native ("Objective-C") counterpart lived on. When a notification was finally delivered, the native object tried to call into the now dead C# object and crashed your App.
To circumvent this, keep a reference to your CustomPaymentObserver e.g. in your AppDelegate to keep it alive.
I'm not sure this is documented somewhere for Xamarin.iOS (couldn't find something on a quick look), but I believe very similar for Xamarin.Droid (e.g. http://developer.xamarin.com/guides/android/advanced_topics/garbage_collection/)

Answer 2

I'm not really sure what was causing this. I thought it was perhaps something related to the linker. But I uninstalled Xamarin completely from my Mac (including all the MonoTouch stuff) and reinstalled everything and now it's working.