If you want to comment out a XAML attribute, follow the following scheme to complete this:

1. Import the Open XML markup compatibility elements:

   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

2. Add another xmlns statement mapping a prefix of your choice (like “ignore”) to an URI of your choice. This URI does not need to point to anything on the web. This is just a unique resource identifier.

   xmlns:ignore="http://something.com/ignore"

3. Use the mc:Ignorable property to set the new prefix as ignorable.

   mc:Ignorable="ignore"

   Note: If you already had one ignorable prefix defined (for example the “d” prefix that Expression Blend and the Visual Studio designer use), no problems. Just add the new prefix to the Ignorable list with a space to separate the prefixes.

   mc:Ignorable="d ignore"

(Source)

If there is an assembly which was not created as a WCF assembly you cannot add for example a new WPF Window with the “Add new Item”. To solve this, the assembly need to be marked as a “WPF”-assembly.

Follow the following steps to complete this:

1. Edit the .proj file
2. Add {60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} into the first property group

So the final .proj file looks someting like:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{2842585C-0E4F-4A4A-92D6-FCC062A3C0F8}</ProjectGuid>
    <OutputType>WinExe</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>ArxWidgetManager</RootNamespace>
    <AssemblyName>ArxWidgetManager</AssemblyName>
    <TargetFrameworkVersion>v4.5.1</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
    <WarningLevel>4</WarningLevel>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    ...

For completeness sake, here’s a list of some other project types:

(Source)