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Retrieving Persistent Property Values with the ReadProperties Event and the ReadProperty Method

The ReadProperties event fires when a custom control is reinstantiated at some point in the development cycle (the Project where it resides has been retrieved and its container has been instantiated, the developer has just entered run mode from the design mode, or the developer has just returned to design mode from run mode).

Notice that we said that ReadProperties fires when the custom control is re-instantiated. We used this phrasing to purposely exclude the case when the developer places an instance of the control on its container for the first time from the Toolbox. For such first-time instantiation, the ReadProperties event doesn't fire. Instead, the InitProperties event fires (see "Using the InitProperties Event to Set Default Starting Property Values"). The ReadProperties event, as its name implies, is the event that you will use to restore the values of properties that have been kept in the Property Bag. The Property Bag appears in the ReadProperties event procedure as a parameter named PropBag. You call PropBag's ReadProperty method for each property whose value you wish to restore, as in Listing 13.9.

Notice that the ReadProperty method takes two arguments: the name of the property as a string and then a default value for the property (in case the property's value has not been initialized in the Property Bag).

We store the results of each call to ReadProperties in the appropriate variable or control property that implements the property within this control.

LISTING 13.9
USING THE READPROPERTIES EVENT PROCEDURE TO RESTORE PERSISTENT PROPERTY VALUES FROM THE PROPERTY BAG

Private Sub UserControl_ReadProperties _
   (PropBag As PropertyBag)
      m_Celsius = PropBag.ReadProperty("Celsius", 30)
      m_TemperatureDate = _
         PropBag.ReadProperty _
      ("TemperatureDate", DateSerial(1997, 1, 1))
      m_caption = PropBag.ReadProperty _
         ("Caption", Extender.Name)
      BackColor = PropBag.ReadProperty _
         ("BackColor", Ambient.BackColor)
      'make housekeeping adjustments
      'to bring constituent controls
      'into line with these property values
      lblCaption.Caption = m_caption
End Sub


  

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