WPF - 为什么不能往Library的工程中添加WPF window

项目中添加一个Library 工程，但是却无法加入WPF window, WPF customize control.

调查了一下，发现这一切都由于Library工程中没有：ProjectTypeGuids

<?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>{072127C4-0CED-4FA8--7A014E1BDF73}</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>Honeywell.OSM.AircraftManagement</RootNamespace>
    <AssemblyName>Honeywell.OSM.AircraftManagement</AssemblyName>
    <TargetFrameworkVersion>v4.</TargetFrameworkVersion>
    <FileAlignment></FileAlignment>
    <ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
    <TargetFrameworkProfile>Client</TargetFrameworkProfile>
  </PropertyGroup>

用Notepad++打开工程，加入ProjectTypeGuids，一切正常了。