VB6 Default Instance Creation Delayed in Windows 10 1607+ Causes NullReferenceException on Form Property Access

Answer to the Core Question

On Windows 10 Anniversary Update (1607) and newer, the VB6 runtime postpones creation of a form’s default instance until the first operation that forces a window handle to be created. If your code accesses a form property that depends on the handle (e.g., Caption, Width, Height) before calling Show, the default instance is still Nothing and a NullReferenceException is thrown.

Confirmed Behavior

• Runtime postpones default‑instance creation until a handle‑dependent operation is invoked.
• Accessing a property that requires the handle before Show triggers the exception.
• Stack traces that reference the form’s handle‑creation routine (e.g., CreateWnd or DoCreate) are a reliable indicator of this delay.

Differentiating from Other Null References

• If the exception occurs immediately after the first property access and no Show call has been made, it is almost certainly due to the delayed instance creation.
• Other null‑reference failures usually appear at a different point in the call stack and are not tied to handle‑creation code.
• Check deployment logs for a stack trace that includes FormClass and a handle‑creation function; this pattern distinguishes the delay‑related error.

Workarounds and Compatibility Settings

• There is no official runtime switch or manifest tag that restores pre‑1607 immediate creation.
• Running the application in Windows 7 compatibility mode (via the Compatibility tab or <compatibility> tags) often forces the older runtime behavior, but this is not guaranteed across all Windows builds.
• When using compatibility mode, verify that other UI aspects (DPI scaling, control rendering) remain acceptable for your users.

Diagnostic Checklist

1. Create a minimal VB6 project that accesses a handle‑dependent property before Show.
   • Run it on Windows 10 1607+ and observe if a NullReferenceException occurs.
   • Capture the stack trace; look for references to CreateWnd or similar.
2. Inspect deployment logs for the same exception pattern.
   • Confirm that the error happens immediately after the first property access and before any Show call.
3. Enable Windows 7 compatibility mode for the executable.
   • Re‑run the test; if the exception disappears, the delay has been mitigated.
4. Review the code to ensure no other null references are possible; the delay is only one potential source.

Next Diagnostic Question

Do you have any other places in the code where a form instance might be accessed without first ensuring it has been created (e.g., through a global variable or a property that returns the default instance)? If so, those could also contribute to null‑reference errors and should be examined separately.