PRB: Accessing Property in Initialize Event Causes Infinite Loop (171979)

SYMPTOMS

If a property of an object is referred to in its own Initialize event, then Visual Basic will fire the Initialize event for that object so that it can be instantiated. An infinite recursion will result until stack space runs out and you receive the following error:

Run-time error '28':
Out of stack space

This will occur for "built-in" objects, such as Forms, and objects created using Visual Basic classes.

CAUSE

Before a property of an object can be accessed, the object must be initialized. This is, in fact, the whole point of the Initialize event; it allows the developer to place the object in a satisfactory state by initializing private data before the properties and methods of an object are invoked. The properties and methods may have to rely on the internal data of the object being in a stable state.

Consequently, the Initialize event must be completed before a property or method of an object can be accessed, and therefore it is not unexpected that the Initialize event is fired again when a property is referred to in the Initialize event.

RESOLUTION

If you are trying to set a property of a Visual Basic built-in object, then it may be possible to refer to the property in an event that occurs later than the Initialize event. For example, rather than setting the Caption of a Form in the Initialize event, set it in the Load event or the Activate event.

An alternative to referring directly to Form1, which is a global object pointer, is to use the Me keyword. When referring directly to Form1 in the Initialize event of the Form, the global object Form1 has not been created so Visual Basic tries to instantiate it. This process causes the infinite recursion. Use the Me Keyword to avoid this recursion.

If you are trying to set a property for an object created with class modules in Visual Basic, it may be appropriate to create a Method for initializing data or setting initial values for properties, for which the client program would be responsible for calling after instantiating an object. (Perhaps a good analogous example of this is the OLE API function OLEInitialize, which client programs written in C must explicitly call before using OLE services).

Another way to accomplish this is to set the private data on which a property relies in the Initialize event. For example, if a Visual Basic property of a class uses a private data member like so:

   Private lPrivProp1 As Long

   Public Property Get lProp1() As Long
   lProp1 = lPrivProp1
   End Property

   Public Property Let lProp1(vNewValue As Long)
   lPrivProp1 = vNewValue
   End Property

then in the Initialize event of the class module in which this property is contained, it is not appropriate to try and set the public property of the class, but rather to set the private data member to a value.

STATUS

This is by design.

NOTE: The behavior with Visual Basic versions 5.0 and 6.0 has changed. You no longer receive the run-time error when accessing the property of an object in the Initialize event of a form. However, it is still recommended that you continue to use the Load or Activate events of a Form to access properties of objects instead of the Initialize event.

MORE INFORMATION

Steps To Reproduce the Problem

Start a new project in Visual Basic. Form1 is created by default.
In the Initialize event of Form1, add the following line of code:
```
      Form1.Caption = "Hello world"
						
```
Run the project by pressing the F5 key. Eventually Visual Basic will run out of stack space.

Steps to Work Around the Problem

Start a new project in Visual Basic. Form1 is created by default.
In the Initialize event, place the following line of code:
```
      Me.Caption = "Hello world"
						
```
Run the project by pressing the F5 key. The Form should appear with a "Hello World" caption.