Issue with ToolTip bound to a dependency property

I am facing an issue with ToolTips (again!)..

My code is as follows:

Xaml file:

<Grid>
        <Button Height="23" Margin="82,0,120,105" Name="button1" VerticalAlignment="Bottom" ToolTip="{Binding Path=Label, Mode=Default}">Button</Button>        
    </Grid>

cs file:

/// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1开发者_开发百科 : Window
    {
        Parameter p1;
        System.Timers.Timer aTimer;

        public Window1()
        {
            InitializeComponent();
            p1 = new Parameter();
            p1.Label = "One thing";
            this.DataContext = p1;

            aTimer = new System.Timers.Timer();
            aTimer.Elapsed += new System.Timers.ElapsedEventHandler(aTimer_Elapsed);
            aTimer.Interval = 5000;
            aTimer.Enabled = true;
        }

        void aTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            aTimer.Enabled = false;
            p1.Label = null;
        }        

    }

The Parameter class is as follows:

class Parameter : System.ComponentModel.INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Members

        public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;

        #endregion

        private string label = String.Empty;

        public string Label
        {
            get { return label; }
            set
            {
                label = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Label"));
            }
        }

        private void OnPropertyChanged(PropertyChangedEventArgs propertyChangedEventArgs)
        {
            try
            {
                if (PropertyChanged != null)
                {
                    PropertyChanged(this, propertyChangedEventArgs);
                }
            }
            catch (Exception exc)
            {
            }
        }
    }

Now after the button is clicked, I get the tooltip "One thing" but after 5 seconds, I get an empty tooltip for sometime. Since I set the dependency property to null, I had expected no tooltip.

Afterwards, if I hover the mouse over the button, I get no empty ToolTip (as expected). Its only during the change of value I get the empty ToolTip.

Can you please help.

Try this... change your Label property and _label variable to type object instead of string i.e. use it as string but declare it as an object.

I guess the problem above happens because null string is actually string.Empty and ToolTip is of object type. The boxing that happens assumes null string as string.empty and hence shows a empty tooltip instead of no tooltip. But if Label is object tyupe then null value will represent no ToolTip.

If the suggested data type change of the Label property is not allowed in your case then then use a Converter in the Label binding to return null object for empty string.

Let me know if this helps.