您是否缺少using指令或程序集引用？C＃错误

我正在尝试创建一个非常简单的C＃程序。

这是xaml文件：

<Window x:Class="HelloWPFApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBlock HorizontalAlignment="Left" Margin="104,72,0,0" TextWrapping="Wrap" Text="Select a message option and then choose the Display button" VerticalAlignment="Top"/>
        <RadioButton x:Name="RadioButton1" Content="Hello;" HorizontalAlignment="Left" Margin="104,138,0,0" VerticalAlignment="Top" Checked="RadioButton1_Checked"/>
        <RadioButton x:Name="RadioButton2" Content="Goodbye;" HorizontalAlignment="Left" Margin="342,138,0,0" VerticalAlignment="Top"/>
        <Button Content="Display" HorizontalAlignment="Left" Margin="211,208,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>

    </Grid>
</Window>

这是.cs文件：

using....
namespace HelloWPFApp
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (RadioButton1.IsChecked == true)
                MessageBox.Show("Hello");
            else
                MessageBox.Show("Goodbye");

        }
    }
}

这是我得到的错误：

'HelloWPFApp.MainWindow'不包含'RadioButton1_Checked'的定义，也找不到扩展方法'RadioButton1_Checked'接受类型为'HelloWPFApp.MainWindow'的第一个参数（您是否缺少using指令或程序集引用？）

你能告诉我问题出在哪里吗？