Use Dim As New to Automatically Instantiate Object

Article ID: Q138533
Creation Date: 23-OCT-1995
Revision Date: 17-NOV-1995

The information in this article applies to:

Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

SUMMARY

When an object is dimensioned "As New," Visual Basic automatically instantiates the object the first time the object variable is used. Furthermore, if the object is set to Nothing, another instance will be instantiated automatically if the object variable is used again.

MORE INFORMATION

To dimension an object "As New," use this syntax:

   Dim MyObject As New MyClass

In this case, the first time MyObject is used, an object of MyClass is instantiated. If you set MyObject to Nothing, the next time it is used a new object of MyClass is created. The following code illustrates the behavior:

   Dim MyObject As New MyClass    ' Dimension new MyObject
   MyObject.Name = "Bob"          ' MyObject is instantiated
   Set MyObject = Nothing         ' MyObject is destroyed
   Print MyObject.Name            ' A new object of type MyClass is created

In contrast, if you use a two-step creation process, Visual Basic will not automatically create the object if it is set to Nothing. In the two-step creation process, you first dimension the object, and then set it is "Set As New." For example:

   Dim MyObject As MyClass        ' Dimension MyObject
   Set MyObject = New MyClass     ' Create a new instance of MyClass
   MyObject.Name = "Bob"          ' Set property
   Set MyObject = Nothing         ' Destroy MyObject
   Print MyObject.Name            ' Error -- Object not set

In this case, you receive an error when using an object that is not set instead of a new object being instantiated automatically. Using the two- step creation process makes your code more readable and easier to debug because it requires explicit instantiation of objects.

Step-by-Step Example

The following Example illustrates the behavior using the built-in collection object.

Start a new project.
Put the following code in the Load event of the default form:
Dim x As New Collection x.Add 1, "One" Debug.Print x.Count Set x = Nothing Debug.Print x.Count ' Creates a New Collection
Dim y As Collection Set y = New Collection y.Add 1, "One" Debug.Print y.Count Set y = Nothing Debug.Print y.Count ' Error -- Object not set
Run the project. An error should occur on the last line illustrating the difference between the one-step creation process that provides automatic instantiation and the two-step creation process that requires explicit instantiation of an object.

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