documented feature by Microsoft

Question

for RunOnce, is the ! prefix a documented feature by Microsoft

Answer 1

Thanks for reaching out.
Yes. The ! prefix for RunOnce registry values is a documented Microsoft feature.

According to Microsoft documentation, when a RunOnce registry entry name starts with !, Windows does not delete the value before execution. This ensures the entry is removed only after the command runs successfully. If the command fails, the entry remains and will be retried on the next logon or startup.

This behavior is documented for the following registry paths:

• HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce
• HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunOnce

Summary of behavior:

• RunOnce without ! → entry is removed before execution
• !RunOnce → entry is removed after successful execution

This is intended for scenarios where reliability is important and the operation must retry if it fails.

References:

• Microsoft Run and RunOnce Registry Keys documentation
• Microsoft Q&A and Windows Driver documentation on RunOnce behavior.

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Answer 2

Yes, the ! prefix is a documented feature by Microsoft for the RunOnce registry keys. It is used to defer the deletion of the RunOnce key until after the command runs successfully. If the RunOnce operation fails and the key does not have the ! prefix, the associated program will not be executed the next time the system starts. This behavior is outlined in the documentation regarding the RunOnce registry key.

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References:

• RunOnce Registry Key
• Run and RunOnce Registry Keys