'System.Windows.Markup.XamlParseException'

Question

I'm new to programming for Windows Phone 8 and I'm working on a simple notepad. I can't seem to figure out this error.

I want to enable a save button when a textbox is no longer empty. I used a keyup event on the textbox itself:

 <TextBox x:Name="TextBoxNote" KeyUp="TextBoxNote_KeyUp" />

Code behind:

 private void TextBoxNote_KeyUp(object sender, System.Windows.Input.KeyEventArgs e)
 {
    if (TextBoxNote.Text == "")
    {
        ApplicationBarSaveButton.IsEnabled = false;
    }
    else
    {
        ApplicationBarSaveButton.IsEnabled = true;
    }
 }

In XAML I set IsEnabled to false

<shell:ApplicationBarIconButton x:Name="ApplicationBarSaveButton"
           IconUri="/Assets/AppBar/e105-Save.76 (1).png"
           Text="save" IsEnabled="False"
           Click="ApplicationBarIconButton_Click_1" />

If I enter something in the textbox, I get this error:

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Notepad
  StackTrace:
       at Notepad.WriteNote.TextBoxNote_KeyUp(Object sender, KeyEventArgs e)
       at MS.Internal.CoreInvokeHandler.InvokeEventHandler(Int32 typeIndex, Delegate handlerDelegate, Object sender, Object args)
       at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)
  InnerException: 

On line:

ApplicationBarSaveButton.IsEnabled = true;

Any Ideas on this one?

Answer 1

Try this,

 private void TextBoxNote_KeyUp(object sender, System.Windows.Input.KeyEventArgs e)
    {
        ApplicationBarIconButton button= (ApplicationBarIconButton)ApplicationBar.Buttons[0];

        if (TextBoxNote.Text == "")
        {
            button.IsEnabled = false;
        }
        else
        {
            button.IsEnabled = true;
        }
    }