Windows has a list of reserved filenames like CON, PRN, AUX, NUL that cannot be created using standard APIs. Learn how to handle them.

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Windows reserves certain filenames (CON, PRN, AUX, NUL, COM0-COM9, LPT0-LPT9) inherited from DOS for backward compatibility. These names are case-insensitive and cannot be used with extensions either. While Win32 APIs enforce this restriction, the underlying file systems (NTFS, FAT32, ReFS) do not — meaning WSL or the `\?\` path prefix can bypass it. In .NET, accessing such filenames via standard System.IO APIs throws an IOException, and Path.GetInvalidFileNameChars() does not flag them. Using the `\?\` prefix in .NET allows creating, reading, and deleting files with reserved names. The Meziantou.Framework.FullPath NuGet package handles this automatically by applying the prefix internally when reserved names are detected.

Reserved filenames on Windows (CON, PRN, AUX, NUL...)