[C] Binding can't convert to valueTypes (#4453)

Binding type coverter wasn't chacking for valuetypes on null inputs.

- fixes #4446

Xamarin.Forms.Core/BindingExpression.cs

@@ -418,7 +418,7 @@ namespace Xamarin.Forms
 		internal static bool TryConvert(ref object value, BindableProperty targetProperty, Type convertTo, bool toTarget)
 		{
 			if (value == null)
-				return true;
+				return !convertTo.GetTypeInfo().IsValueType;
 			if ((toTarget && targetProperty.TryConvert(ref value)) || (!toTarget && convertTo.IsInstanceOfType(value)))
 				return true;
 

Xamarin.Forms.Core/TypedBinding.cs

@@ -5,6 +5,7 @@ using System.ComponentModel;
 using System.Globalization;
 using System.Collections.Generic;
 using Xamarin.Forms.Internals;
+using System.Reflection;
 
 namespace Xamarin.Forms.Internals
 {
@@ -287,4 +288,4 @@ namespace Xamarin.Forms.Internals
 				_handlers [i].Listener.Unsubscribe();
 		}
 	}
-}
+}

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4446.xaml

@@ -0,0 +1,10 @@
+<?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.Gh4446">
+	<StackLayout>
+		<Label x:Name="label" IsVisible="{Binding Text}" x:DataType="local:Gh4446Item" />
+	</StackLayout>
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4446.xaml.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Globalization;
+using NUnit.Framework;
+
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+    public class Gh4446Item
+    {
+        public string Id { get; set; }
+        public string Text { get; set; }
+        public string Description { get; set; }
+    }
+
+	public partial class Gh4446 : ContentPage
+	{
+		public Gh4446()
+		{
+			InitializeComponent();
+		}
+
+		public Gh4446(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 BindingThrowsOnWrongConverterParameter(bool useCompiledXaml)
+			{
+				Assert.DoesNotThrow(() => new Gh4446(useCompiledXaml) { BindingContext = new Gh4446Item { Text = null } });
+			}
+		}
+	}
+}