Is there a need to set Objects to Nothing

ghz 7months ago ⋅ 100 views

I always read that it is recommended to set objects to nothing, once I am done with them. But I normally use them only in functions inside forms.

Isn't the reference lost and memory released when the function scope is left, regardless of setting objects to Nothing?

i.e. is it really necessary to do:

Set db = Nothing
Set record_set = Nothing

Answers

In VB6, it is a common practice to explicitly set object references to Nothing when you are done with them. However, understanding the necessity and implications of this practice requires a closer look at how VB6 handles object references and memory management.

Automatic Cleanup by Scope

When an object variable goes out of scope (e.g., a function or procedure ends), VB6 automatically releases the reference to the object. This means that in many cases, explicitly setting the object to Nothing is not strictly necessary. Here’s an example:

Private Sub ExampleProcedure()
    Dim db As Database
    Dim record_set As Recordset

    ' Set up and use the objects
    Set db = OpenDatabase("mydatabase.mdb")
    Set record_set = db.OpenRecordset("SELECT * FROM mytable")

    ' Use the recordset
    ' ...

    ' Explicitly setting to Nothing
    Set record_set = Nothing
    Set db = Nothing
End Sub

When to Set Objects to Nothing

Despite automatic cleanup, there are situations where explicitly setting objects to Nothing is beneficial:

  1. Immediate Release: If you want to release the resources immediately rather than waiting for the variable to go out of scope, especially in long-running procedures.
  2. Circular References: In cases where objects have circular references, setting objects to Nothing can help break the reference cycle and ensure proper cleanup.
  3. Resource-Intensive Objects: For objects that consume significant resources (e.g., database connections, file handles), explicitly setting them to Nothing can free up resources promptly.
  4. Error Handling: In complex procedures with multiple exit points, ensuring that objects are set to Nothing can help avoid resource leaks if the procedure exits unexpectedly.

Best Practices

While VB6 will handle most object cleanup automatically when they go out of scope, following best practices can improve the robustness and reliability of your code:

  1. Explicit Cleanup: For critical resources, explicitly set objects to Nothing.
  2. Consistent Style: Adopt a consistent style in your code to enhance readability and maintainability.
  3. Use Error Handling: Incorporate error handling to ensure that objects are properly cleaned up in case of errors.

Example with Error Handling

Here’s an enhanced example with error handling and explicit cleanup:

Private Sub ExampleProcedure()
    On Error GoTo Cleanup

    Dim db As Database
    Dim record_set As Recordset

    ' Set up and use the objects
    Set db = OpenDatabase("mydatabase.mdb")
    Set record_set = db.OpenRecordset("SELECT * FROM mytable")

    ' Use the recordset
    ' ...

Cleanup:
    ' Ensure objects are cleaned up in case of an error
    If Not record_set Is Nothing Then
        record_set.Close
        Set record_set = Nothing
    End If
    If Not db Is Nothing Then
        db.Close
        Set db = Nothing
    End If
    If Err.Number <> 0 Then
        ' Handle the error (log it, display a message, etc.)
        MsgBox "An error occurred: " & Err.Description
    End If
End Sub

Conclusion

While VB6’s automatic cleanup often handles object references when they go out of scope, explicitly setting objects to Nothing can provide more immediate resource release, prevent memory leaks in specific scenarios, and enhance code clarity. It is a good practice, particularly for resource-intensive objects or in procedures with complex control flows and error handling.

#VB6