程序代码：

class ValueChangedEventArgs : EventArgs
{
    public string OldValue { get; private set; }
    public string NewValue { get; private set; }

    public ValueChangedEventArgs(string oldValue, string newValue)
    {
        OldValue = oldValue;
        NewValue = newValue;
    }
}

class ObservableString
{
    public EventHandler<ValueChangedEventArgs> ValueChanged = delegate { };

    private string value = null;

    public string Value
    {
        get
        {
            return value;
        }

        set
        {
            if (this.value != value)
            {
                string oldValue = this.value;

                this.value = value;

                ValueChanged(this, new ValueChangedEventArgs(oldValue, value));
            }
        }
    }
}

static void Main()
{
    ObservableString s = new ObservableString();

    s.ValueChanged += (sender, args) =>
        {
            Console.WriteLine("Value has changed from '{0}' to '{1}'!", args.OldValue, args.NewValue);
        };

    s.Value = "Some text";
    s.Value = "Some other text";
    s.Value = "More text";

    Console.ReadLine();
}