INotifyPropertyChanged in UserControl

I have a custom control which is inherited from TextBox control. I would like to implement the INotifyPropertyChanged interface in my custom control.

public class CustomTextBox : TextBox, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected void NotifyPropertyChanged(string info)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(info));
    }
}

My problem is when I try to raised a PropertyChanged event the PropertyChanged event handler is always null.

Anybody can help me?