WPF中‘UserControl’命名空间被当作类型使用的错误解决

错误原因 #

你的CardDay类所在的命名空间是WeatherApp.UserControl，而WPF内置的控件类型也叫UserControl（来自System.Windows.Controls命名空间），编译器被同名的命名空间和类型搞混了，把你要继承的UserControl当成了当前的命名空间，所以抛出错误。

两种解决办法 #

办法1：重命名命名空间（推荐） #

把命名空间改成和控件类型不冲突的名字，比如WeatherApp.UserControls（加个s），同时同步修改XAML里的类路径。

修正后的后端代码（CardDay.xaml.cs）：

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace WeatherApp.UserControls
{
    public partial class CardDay : UserControl
    {
        public CardDay()
        {
            InitializeComponent();
        }

        public string Day
        {
            get { return (string)GetValue(DayProperty); }
            set { SetValue(DayProperty, value); }
        }
        public static readonly DependencyProperty DayProperty = DependencyProperty.Register("Day", typeof(string), typeof(CardDay));

        public string MaxNum
        {
            get { return (string)GetValue(MaxNumProperty); }
            set { SetValue(MaxNumProperty, value); }
        }
        public static readonly DependencyProperty MaxNumProperty = DependencyProperty.Register("MaxNum", typeof(string), typeof(CardDay));

        public string MinNum
        {
            get { return (string)GetValue(MinNumProperty); }
            set { SetValue(MinNumProperty, value); }
        }
        public static readonly DependencyProperty MinNumProperty = DependencyProperty.Register("MinNum", typeof(string), typeof(CardDay));

        public ImageSource Source
        {
            get { return (ImageSource)GetValue(SourceProperty); }
            set { SetValue(SourceProperty, value); }
        }
        public static readonly DependencyProperty SourceProperty = DependencyProperty.Register("Source", typeof(ImageSource), typeof(CardDay));
    }
}

修正后的XAML代码（CardDay.xaml）：

<UserControl x:Class="WeatherApp.UserControls.CardDay"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" Name="cardDay">
    <Border CornerRadius="10" Width="85" Padding="10" Margin="0 0 10 0" BorderThickness="1">
        <Border.Style>
            <Style TargetType="Border">
                <Setter Property="Background" Value="White"/>
                <Setter Property="BorderBrush" Value="#e9e9e9e9"/>

                <Style.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter Property="BorderBrush" Value="#03a9f4"/>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </Border.Style>
        <StackPanel>
            <TextBlock Text="{Binding Path=Day, ElementName=cardDay}" TextAlignment="Center" FontSize="14" FontWeight="SemiBold"/>
            <Image Source="{Binding Path=Source, ElementName=cardDay}" Width="30" Margin="0 10 0 10"/>
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
                <TextBlock Text="{Binding Path=MaxNum, ElementName=cardDay}" TextAlignment="Center" FontWeight="SemiBold" Margin="0 0 8 0"/>
                <TextBlock Text="{Binding Path=MinNum, ElementName=cardDay}" TextAlignment="Center" FontWeight="SemiBold" Background="#7a7a7a"/>
            </StackPanel>
        </StackPanel>
    </Border>
</UserControl>

办法2：使用完全限定类型名 #

如果不想修改命名空间，直接在继承时写出UserControl的完整命名空间路径，明确告诉编译器你要引用的是控件类型：

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace WeatherApp.UserControl
{
    // 这里用System.Windows.Controls.UserControl代替UserControl
    public partial class CardDay : System.Windows.Controls.UserControl
    {
        public CardDay()
        {
            InitializeComponent();
        }

        // 后面的依赖属性代码保持不变
        public string Day
        {
            get { return (string)GetValue(DayProperty); }
            set { SetValue(DayProperty, value); }
        }
        public static readonly DependencyProperty DayProperty = DependencyProperty.Register("Day", typeof(string), typeof(CardDay));

        public string MaxNum
        {
            get { return (string)GetValue(MaxNumProperty); }
            set { SetValue(MaxNumProperty, value); }
        }
        public static readonly DependencyProperty MaxNumProperty = DependencyProperty.Register("MaxNum", typeof(string), typeof(CardDay));

        public string MinNum
        {
            get { return (string)GetValue(MinNumProperty); }
            set { SetValue(MinNumProperty, value); }
        }
        public static readonly DependencyProperty MinNumProperty = DependencyProperty.Register("MinNum", typeof(string), typeof(CardDay));

        public ImageSource Source
        {
            get { return (ImageSource)GetValue(SourceProperty); }
            set { SetValue(SourceProperty, value); }
        }
        public static readonly DependencyProperty SourceProperty = DependencyProperty.Register("Source", typeof(ImageSource), typeof(CardDay));
    }
}

内容的提问来源于stack exchange，提问作者FahadShaheb