[C] elements from Source are like items (#13490) fixes #13209

treat elements from the "Source" RD like if they were part of the actual
dictionary, for Count, and Remove

Xamarin.Forms.Core.UnitTests/ResourceDictionaryTests.cs

@@ -345,7 +345,7 @@ namespace Xamarin.Forms.Core.UnitTests
 		}
 
 		[Test]
-		public void CountDoesNotIncludeMerged()
+		public void CountDoesIncludeMerged()
 		{
 			var rd = new ResourceDictionary {
 				{"baz", "Baz"},
@@ -353,7 +353,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			};
 			rd.MergedWith = typeof(MyRD);
 
-			Assert.That(rd.Count, Is.EqualTo(2));
+			Assert.That(rd.Count, Is.EqualTo(4));
 		}
 
 		[Test]

Xamarin.Forms.Core/ResourceDictionary.cs

@@ -175,7 +175,7 @@ namespace Xamarin.Forms
 
 		public int Count
 		{
-			get { return _innerDictionary.Count; }
+			get { return _innerDictionary.Count + (_mergedInstance?.Count ?? 0); }
 		}
 
 		bool ICollection<KeyValuePair<string, object>>.IsReadOnly
@@ -230,7 +230,7 @@ namespace Xamarin.Forms
 
 		public bool Remove(string key)
 		{
-			return _innerDictionary.Remove(key);
+			return _innerDictionary.Remove(key) || (_mergedInstance?.Remove(key) ?? false);
 		}
 
 		public ICollection<object> Values

Xamarin.Forms.Xaml.UnitTests/AppResources/RD13209.xaml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" >
+	<Color x:Key="Color1">Chartreuse</Color>
+</ResourceDictionary>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh13209.xaml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<ContentPage
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    x:Class="Xamarin.Forms.Xaml.UnitTests.Gh13209">
+    <StackLayout x:Name="Root">
+		<StackLayout.Resources>
+			<ResourceDictionary Source="../AppResources/RD13209.xaml"/>
+		</StackLayout.Resources>
+		<Rectangle x:Name="MyRect" BackgroundColor="{StaticResource Color1}" WidthRequest="100" HeightRequest="100" Margin="6"/>
+		<Label x:Name="MyLabel" WidthRequest="200" HeightRequest="30" Margin="6"/>
+	</StackLayout>
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh13209.xaml.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public partial class Gh13209 : ContentPage
+	{
+		public Gh13209() => InitializeComponent();
+		public Gh13209(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 RdWithSource(bool useCompiledXaml)
+			{
+				var layout = new Gh13209(useCompiledXaml);
+				Assert.That(layout.MyRect.BackgroundColor, Is.EqualTo(Color.Chartreuse));
+				Assert.That(layout.Root.Resources.Count, Is.EqualTo(1));
+				Assert.That(layout.Root.Resources.MergedDictionaries.Count, Is.EqualTo(0));
+
+				Assert.That(layout.Root.Resources["Color1"], Is.Not.Null);
+				Assert.That(layout.Root.Resources.Remove("Color1"), Is.True);
+				Assert.Throws<KeyNotFoundException>(()=>
+				{
+					var _ = layout.Root.Resources["Color1"];
+				});
+
+			}
+		}
+	}
+}