[C] fallback nicely in case of IndexOutOfRangeEx (#4525)

This PR does 3 things:
- In case of Binding path with indexer, lookout for
  IndexOutOfRangeException in addition to KeyNotFoundException.
- in case of KeyNotFound or IndexOutOfRange, use the FallbackValue if
  any.
- in case of an uncaught exception, throw that exception instead of a
  TargetInvocationException.

- fixes #4516

Xamarin.Forms.Core/BindingExpression.cs

@@ -610,15 +610,16 @@ namespace Xamarin.Forms
 				{
 					if (IsIndexer)
 					{
-						try
+						try {
-						{
 							value = LastGetter.Invoke(value, Arguments);
 						}
-						catch (TargetInvocationException ex)
+						catch (TargetInvocationException ex) {
-						{
+							if (ex.InnerException is KeyNotFoundException || ex.InnerException is IndexOutOfRangeException) {
-							if (!(ex.InnerException is KeyNotFoundException))
+								value = null;
-								throw;
+								return false;
-							value = null;
+							}
+							else
+								throw ex.InnerException;
 						}
 						return true;
 					}

Xamarin.Forms.Core/TypedBinding.cs

@@ -197,7 +197,7 @@ namespace Xamarin.Forms.Internals
 				if (isTSource) {
 					try {
 						value = GetSourceValue(_getter((TSource)sourceObject), property.ReturnType);
-					} catch (Exception ex) when (ex is NullReferenceException || ex is KeyNotFoundException) {
+					} catch (Exception ex) when (ex is NullReferenceException || ex is KeyNotFoundException || ex is IndexOutOfRangeException) {
 					}
 				}
 				if (!BindingExpression.TryConvert(ref value, property, property.ReturnType, true)) {

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4516.xaml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage
+		xmlns="http://xamarin.com/schemas/2014/forms"
+		xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+		xmlns:local="using:Xamarin.Forms.Xaml.UnitTests"
+		x:Class="Xamarin.Forms.Xaml.UnitTests.Gh4516">
+	<Image x:Name="image" Source="{Binding Images[0], FallbackValue='foo.jpg'}" />
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4516.xaml.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public class Gh4516VM {
+		public Uri[] Images { get; } = { };
+	}
+
+	public partial class Gh4516 : ContentPage
+	{
+		public Gh4516() => InitializeComponent();
+		public Gh4516(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture]
+		class Tests
+		{
+			[SetUp] public void Setup() => Device.PlatformServices = new MockPlatformServices();
+			[TearDown] public void TearDown() => Device.PlatformServices = null;
+
+			[TestCase(true), TestCase(false)]
+			public void BindingToEmptyCollection(bool useCompiledXaml)
+			{
+				Gh4516 layout = null;
+				Assert.DoesNotThrow(() => layout = new Gh4516(useCompiledXaml) { BindingContext = new Gh4516VM() });
+				Assert.That((layout.image.Source as FileImageSource).File, Is.EqualTo("foo.jpg"));
+			}
+		}
+	}
+}