example.cs

public IEnumerable<string> ReadFileLineByLine(string fileName)
{
    if (string.IsNullOrEmpty(fileName))
    {
        throw new ArgumentNullException(nameof(fileName));
    }

    return ReadFileLineByLineImpl();

    IEnumerable<string> ReadFileLineByLineImpl()
    {
        foreach (var line in File.ReadAllLines(fileName))
        {
            yield return line;
        }
    }
}