Bug Description

In GenerateUniqueFileName(), Path.GetDirectoryName() can return null (e.g. for root paths like C:\ or relative filenames without a directory component). The null-forgiving operator ! suppresses the compiler warning but does not prevent a NullReferenceException at runtime if directory is subsequently used in path operations.

Actual Behavior

// RotatingStreamWriter.cs lines 241-242
string directory = Path.GetDirectoryName(basePath)!;
string fileName = Path.GetFileName(basePath);

If basePath is a root path or a bare filename (e.g. "output.log" without directory), GetDirectoryName returns null or "", and downstream Path.Combine(directory, ...) will produce unexpected results or throw.

Suggested Fix

Handle the null case explicitly with a fallback to the current directory:

string directory = Path.GetDirectoryName(basePath) ?? string.Empty;
string fileName = Path.GetFileName(basePath);

Or validate early and throw a descriptive error:

string? directory = Path.GetDirectoryName(basePath);
if (string.IsNullOrEmpty(directory))
    throw new ArgumentException($"Cannot determine directory from path: {basePath}", nameof(basePath));

Environment

• File: src/Servy.Core/IO/RotatingStreamWriter.cs
• Line: 241