Replace XmlnsSupportsValidationAttribute (#15855)

* Replace XmlnsSupportsValidationAttribute

* Add references

* Remove Microsoft.Windows.Design.Extensibility ref

* Revert "Remove Microsoft.Windows.Design.Extensibility ref"

This reverts commit 5a24df3.

* Testing something out

* Update Xamarin.Forms.Core.Design.csproj

* Update Xamarin.Forms.Core.Design.csproj

* PR Feedback

.gitignore

@@ -75,3 +75,4 @@ Visual Studio 2019/**
 TestResult.xml
 build/packages
 WriteDevopsVariables.csproj
+!Microsoft.VisualStudio.DesignTools.Extensibility.dll

Xamarin.Forms.Core.Design/AttributeTableBuilder.cs

@@ -1,7 +1,6 @@
 using System;
 using System.ComponentModel;
 using System.Linq;
-using System.Windows.Markup;
 using Microsoft.Windows.Design;
 using Xamarin.Forms.Internals;
 
@@ -13,7 +12,7 @@ public AttributeTableBuilder()
 		{
 			// Turn off validation of values, which doesn't work for OnPlatform/OnIdiom
 			AddCustomAttributes(typeof(AbsoluteLayout).Assembly,
-				new XmlnsSupportsValidationAttribute("http://xamarin.com/schemas/2014/forms", false));
+				new Microsoft.VisualStudio.DesignTools.Extensibility.XmlnsSupportsValidationAttribute("http://xamarin.com/schemas/2014/forms", false));
 
 			// Style isn't a view, make it visible
 			AddCallback(typeof(Style), builder => builder.AddCustomAttributes(

Xamarin.Forms.Core.Design/Xamarin.Forms.Core.Design.csproj

@@ -79,6 +79,12 @@
     <Compile Include="KeyboardDesignTypeConverter.cs" />
     <Compile Include="EasingDesignTypeConverter.cs" />
   </ItemGroup>
+  <ItemGroup>
+    <Reference Include="Microsoft.VisualStudio.DesignTools.Extensibility, Version=17.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
+      <SpecificVersion>False</SpecificVersion>
+      <HintPath>..\Xamarin.Forms.Xaml.Design\Microsoft.VisualStudio.DesignTools.Extensibility.dll</HintPath>
+    </Reference>
+  </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <!-- Ensure that all images in the 'mac' and 'win' subdirectories are included as embedded resources -->
   <!-- using a defined format. That format is "{platform}.{imagename}". We will look up images using   -->

Xamarin.Forms.Xaml.Design/AttributeTableBuilder.cs

@@ -1,5 +1,4 @@
 using System;
-using System.ComponentModel;
 using Microsoft.Windows.Design;
 
 namespace Xamarin.Forms.Xaml.Design
@@ -9,7 +8,7 @@ class AttributeTableBuilder : Microsoft.Windows.Design.Metadata.AttributeTableBu
 		public AttributeTableBuilder()
 		{
 			AddCustomAttributes(typeof(ArrayExtension).Assembly,
-				new XmlnsSupportsValidationAttribute("http://xamarin.com/schemas/2014/forms", false));
+				new Microsoft.VisualStudio.DesignTools.Extensibility.XmlnsSupportsValidationAttribute("http://xamarin.com/schemas/2014/forms", false));
 
 			AddCallback(typeof(OnPlatformExtension), builder => builder.AddCustomAttributes(new Attribute[] {
 				new System.Windows.Markup.MarkupExtensionReturnTypeAttribute (),

Xamarin.Forms.Xaml.Design/Xamarin.Forms.Xaml.Design.csproj

@@ -61,5 +61,11 @@
       <Private>False</Private>
     </ProjectReference>
   </ItemGroup>
+  <ItemGroup>
+    <Reference Include="Microsoft.VisualStudio.DesignTools.Extensibility, Version=17.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
+      <SpecificVersion>False</SpecificVersion>
+      <HintPath>.\Microsoft.VisualStudio.DesignTools.Extensibility.dll</HintPath>
+    </Reference>
+  </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
 </Project>