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.
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## Win32 API vs. File System## Creating reserved files with WSL## Accessing reserved files in .NET## Bypassing the restriction## Additional resources5K Impressions