Runtime error 91 not raised: VB6 object variable silently auto-instantiates on member access

Direct Answer

VB6 only auto-instantiates an object variable when it is declared with the As New syntax (Dim obj As New MyClass). A plain Dim obj As MyClass without New never auto-instantiates; any member access on a Nothing reference raises runtime error 91. This rule applies identically to early-bound and late-bound calls, and the presence of a default property (VB_UserMemId = 0) does not alter it.

Confirmed Behavior

• Auto-instantiation trigger: Only the As New declarator. The compiler emits a hidden If obj Is Nothing Then Set obj = New MyClass guard before every member access on such variables.
• Late binding: Dim obj As Object or CallByName on a Nothing reference raises error 91. No silent instantiation occurs.
• Default properties: obj.DefaultProp on a Nothing reference raises error 91 regardless of VB_UserMemId.
• Project options: Option Explicit, optimization settings, VB_PredeclaredId, VB_Exposed — none change the error-91 behavior for non-As New variables.

Why You May See Silent Success

If your code shows Dim obj As MyClass but member access succeeds without error 91, one of these is almost certainly true:

1. The variable is actually declared As New in the saved file (the VB6 designer sometimes serializes As New while the code window shows As MyClass).
2. A base form, module, or inherited template declares the variable As New.
3. A With block or generated code path performs an implicit Set before the call.
4. Binary compatibility cached an older As New declaration; a full clean rebuild (delete *.exp, *.lib, *.obj) is required after editing.

Verification Steps

1. Open the .frm or .cls file in Notepad and search for As New on the variable name.
2. Add Debug.Assert obj Is Nothing immediately before the member access; the IDE breaks if the variable is already instantiated.
3. Compile to native code with No Optimizations and Create Symbolic Debug Info, set a breakpoint on the call, and inspect the Locals window — it will show Nothing or an instance pointer.
4. Install a global error handler that logs Err.Number at every call site to confirm error 91 is raised.

Enforcing Deterministic Error 91

To guarantee uninitialized access fails:

• Never use As New in declarations. Use Dim obj As MyClass and explicit Set obj = New MyClass.
• Add a defensive guard in a wrapper property or initialization helper:
  If obj Is Nothing Then Err.Raise 91
• Consider a code-review rule or static-analysis script that flags As New in variable declarations.

One Diagnostic Detail Needed

Does the variable declaration appear as Dim obj As MyClass in the raw .frm/.cls file (opened in Notepad), or does the file contain Dim obj As New MyClass? This single check determines whether the fix is removing New or investigating a deeper tooling issue.