Windows has reserved filenames, this prevents clashes and problems

ReservedFileNameTool.cs

using System.IO;


/// <summary>
/// Contains logic for detecting reserved file names in Windows.
/// </summary>
static class ReservedFileNameTool
{
	/// <summary>
	/// Reserved file names in Windows.
	/// </summary>
	private static readonly string[] _reserved = new string[]
	{
		"con",
		"prn",
		"aux",
		"nul",
		"com1",
		"com2",
		"com3",
		"com4",
		"com5",
		"com6",
		"com7",
		"com8",
		"com9",
		"lpt1",
		"lpt2",
		"lpt3",
		"lpt4",
		"lpt5",
		"lpt6",
		"lpt7",
		"lpt8",
		"lpt9",
		"clock$"
	};

	/// <summary>
	/// Determine if the path file name is reserved.
	/// </summary>
	public static bool IsReservedFileName(string FilePath)
	{
		string FileName = Path.GetFileNameWithoutExtension(FilePath); // Extension doesn't matter
		FileName = FileName.ToLower(); // Case-insensitive
		foreach (string ReservedName in _reserved)
		{
			if (ReservedName == FileName) return true;
		}
		return false;
	}

	/// <summary>
	/// Determine if the path file name is reserved.
	/// </summary>
	public static bool IsNotReservedFileName(string path)
	{
		return !IsReservedFileName(path);
	}

	public static string FixFilename(string path)
	{
		if (IsReservedFileName(path))
		{
			string parent = Path.GetDirectoryName(path);
			string ext = Path.GetExtension(path);
			string fn = Path.GetFileNameWithoutExtension(path);
			return Path.Combine(parent, $"{fn}-{ext}");
		}
		else
		{
			return path;
		}
	}
}