All.cs

using System.IO;

namespace A
{
    public class B
    {
        private string _configurationFilePath = "config.txt";
        private string _recPath = string.Empty;
        
        public string ReadConfig()
        {
            return File.ReadAllText(_configurationFilePath);
        }
    }
}

LineByLine.cs

using System.IO;

namespace A
{
    public class B
    {
        private string _configurationFilePath = "config.txt";
        private string _recPath = string.Empty;
        
        public IEnumerable<string> ReadConfig()
        {
            return File.ReadLines(_configurationFilePath);
        }
    }
}