[binding] return defaultValueCreator on failure (#4541)

if the FallbackValue isn't set, returns the value created by the
defaultValueCreator instead of the DefaultValue when a binding can not
be resolved.

- fixes #2752

Xamarin.Forms.Core/BindingExpression.cs

@@ -152,12 +152,11 @@ namespace Xamarin.Forms
 			if (needsGetter)
 			{
 				object value = property.DefaultValue;
-				if (part.TryGetValue(current, out value) || part.IsSelf)
-				{
+				if (part.TryGetValue(current, out value) || part.IsSelf) {
 					value = Binding.GetSourceValue(value, property.ReturnType);
 				}
 				else
-					value = Binding.FallbackValue ?? property.DefaultValue;
+					value = Binding.FallbackValue ?? property.GetDefaultValue(target);
 
 				if (!TryConvert(ref value, property, property.ReturnType, true))
 				{

Xamarin.Forms.Core/TypedBinding.cs

@@ -193,7 +193,7 @@ namespace Xamarin.Forms.Internals
 				Subscribe((TSource)sourceObject);
 
 			if (needsGetter) {
-				var value = FallbackValue ?? property.DefaultValue;
+				var value = FallbackValue ?? property.GetDefaultValue(target);
 				if (isTSource) {
 					try {
 						value = GetSourceValue(_getter((TSource)sourceObject), property.ReturnType);

Xamarin.Forms.Xaml.UnitTests/Issues/Gh2752.xaml

@@ -0,0 +1,9 @@
+<?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.Gh2752"
+		x:DataType="local:Gh2752VM"
+		My="{Binding Foo.Bar.Baz}">
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh2752.xaml.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public class Gh2752VM {
+		public Gh2752VM Foo { get; set; }
+		public Gh2752VM Bar { get; set; }
+		public string Baz { get; set; }
+	}
+	public partial class Gh2752 : ContentPage
+	{
+		public static readonly BindableProperty MyProperty =
+			BindableProperty.Create("My", typeof(string), typeof(Gh2752), default(string), defaultValueCreator: b=> "default created value");
+
+		public string My {
+			get { return (string)GetValue(MyProperty); }
+			set { SetValue(MyProperty, value); }
+		}
+
+		public Gh2752() => InitializeComponent();
+		public Gh2752(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 FallbcakToDefaultValueCreator(bool useCompiledXaml)
+			{
+				var layout = new Gh2752(useCompiledXaml) { BindingContext = null };
+				Assert.That(layout.My, Is.EqualTo("default created value"));
+			}
+		}
+	}
+}