Interacting with file system

FileSystemAgent.cs

/// <summary>
/// Agent for interacting with file system
/// </summary>
public class FileSystemAgent : IFileSystemAgent
{
    public IList<string> ReadAllLines(string fileName)
    {
        if (!File.Exists(fileName))
        {
            throw new ArgumentException($"File {fileName} doesn't exist");
        }
        return File.ReadAllLines(fileName).ToList();
    }
    
    public IEnumerable<string> ReadLineByLine(string fileName)
    {
        if (!File.Exists(fileName))
        {
            throw new ArgumentException($"File {fileName} doesn't exist");
        }
        using (var reader = File.OpenText(fileName))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
            {
                yield return line;
            }
        }
    }

    public void WriteAllLines(IEnumerable<string> lines, string fileName)
    {
        using (StreamWriter file = new StreamWriter(fileName))
        {
            foreach (string line in lines)
            {
                file.WriteLine(line);
            }
        }
    }
}