c# - 名称“x”不存在于命名空间“using:..”中

Question

我一直在构建一个简单的 RSS 应用程序，但我遇到了这个问题。

<common:LayoutAwarePage
  x:Name="pageRoot"
  x:Class="App4.View.RssMainView"
  DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="using:App4.View"
  xmlns:common="using:App4.Common"
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:vm="using:App4.ViewModel"
  mc:Ignorable="d">

<Page.Resources>

    <vm:RssMainViewModel x:Key="ViewModel" x:Name="ViewModel" 
                 FeedUrlString="http://blogs.msdn.com/b/juank/rss.aspx"/>

    <!-- TODO: Delete this line if the key AppName is declared in App.xaml -->
    <x:String x:Key="AppName">My Application</x:String>
</Page.Resources>

我的 App4.ViewModel 看起来像这样；

using App4.Common;
using App4.Model;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using App4.Util;

namespace App4.ViewModel
{
    public class RssMainViewModel : BindableBase
    {
        private ArticleList _articles = new ArticleList();
        public ArticleList Articles
        {
            get { return _articles; }
            set
            {
                SetProperty(ref _articles, value);
            }
        }

        private string _feedUrlString;
        public string FeedUrlString
        {
            get { return _feedUrlString; }
            set
            {
                SetProperty(ref _feedUrlString, value);
            }
        }
    }
}

它显示此错误“名称“RssMainView”在命名空间“using:App4.ViewModel”中不存在我该如何解决？