title: Why can't I change this label with this button?
body: <pre><code>using System;  
  using System.Drawing;  
  using System.Windows.Forms;  
  namespace Test  
  {  
      class mainWindow : Form  
      {  
          public mainWindow()  
          {  
              Label firstLabel = new label();  
              firstLabel.Text = "Hello";  
              this.Controls.Add(firstLabel);  
              Button firstButton = new Button();  
              firstButton.Text = "Click me";  
              firstButton.Click += firstButton_Click;  
              firstbutton.Left = 100;  
              this.Controls.Add(firstButton);  
          }  
          void firstButton_Click(object sender, EventArgs e)  
          {  
              firstlabel.Text = "Goodbye";  
          }  
      }  
      class XxX  
      {  
          static void Main()  
          {  
              mainWindow form = new mainWindow();  
              Application.Run(form);  
          }  
      }
  }  
  </code></pre>