title: are you missing a using directive or an assembly reference? Error in C#
body: <p>I'm trying to create a very simple C# program.</p>
  
  <p>Here is the xaml file : </p>
  
  <pre><code>&lt;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"&gt;
      &lt;Grid&gt;
          &lt;TextBlock HorizontalAlignment="Left" Margin="104,72,0,0" TextWrapping="Wrap" Text="Select a message option and then choose the Display button" VerticalAlignment="Top"/&gt;
          &lt;RadioButton x:Name="RadioButton1" Content="Hello;" HorizontalAlignment="Left" Margin="104,138,0,0" VerticalAlignment="Top" Checked="RadioButton1_Checked"/&gt;
          &lt;RadioButton x:Name="RadioButton2" Content="Goodbye;" HorizontalAlignment="Left" Margin="342,138,0,0" VerticalAlignment="Top"/&gt;
          &lt;Button Content="Display" HorizontalAlignment="Left" Margin="211,208,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/&gt;
  
      &lt;/Grid&gt;
  &lt;/Window&gt;
  </code></pre>
  
  <p>Here is the .cs file :</p>
  
  <pre><code>using....
  namespace HelloWPFApp
  {
      /// &lt;summary&gt;
      /// Interaction logic for MainWindow.xaml
      /// &lt;/summary&gt;
      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");
  
          }
      }
  }
  </code></pre>
  
  <p>And this is the error I get : </p>
  
  <blockquote>
    <p>'HelloWPFApp.MainWindow' does not contain a definition for
    'RadioButton1_Checked' and no extension method 'RadioButton1_Checked'
    accepting a first argument of type 'HelloWPFApp.MainWindow' could be
    found (are you missing a using directive or an assembly reference?)</p>
  </blockquote>
  
  <p>Can you tell me what the problem is?</p>