Storing Persistent Property Values with the WriteProperties Event and the WriteProperty Method

Visual Basic fires the UserControl's WriteProperties event just before it fires the UserControl's Terminate event provided that at least one property value has changed. In other words, the WriteProperties event fires whenever the current instance of the control is about to be destroyed and any property values that you want to persist have changed and, therefore, need to be saved.

As its name implies, you use the WriteProperties event procedure to save persistent property values. The specific mechanism you use to save property values is to call the WriteProperty method of the Property Bag for each property whose value you wish to save. The Property Bag is available in the event procedure of the WriteProperties event as a parameter named PropBag. The example code in Listing 13.7 shows how you would call the Property Bag's WriteProperty method to save individual property values. Notice that we use whatever repository has been storing the property value as the source for the current value: at times this might be a private memory variable, and at other times it might be a property of a constituent control (as in the final line before the End Sub).

LISTING 13.7
USING THE WRITEPROPERTIES EVENT PROCEDURE TO SAVE PROPERTY VALUES TO THE PROPERTY BAG

Private Sub UserControl_WriteProperties _
   (PropBag As PropertyBag)
      'Store the values of the custom properties
      'to the Property Bag
      PropBag.WriteProperty _
         "BackColor", BackColor
      PropBag.WriteProperty _
         "Celsius", m_Celsius
      PropBag.WriteProperty _
         "Fahrenheit", m_Fahrenheit
      PropBag.WriteProperty _
         "TemperatureDate", m_TemperatureDate
      PropBag.WriteProperty _
         "Caption", lblCaption.Caption
End Sub