Simple CSV Loader first version

SimpleCsvLoader.cs

using System;
using System.Collections.Generic;
using System.IO;

namespace SimpleCsvLoader
{
    public class CsvLoader
    {
        private char _delimiter;
        private string _fileName;
        private StreamReader _reader;

        private CsvLoader(char delimiter)
        {
            _delimiter = delimiter;
        }

        public CsvLoader(string fileName, char delimiter = ',')
            : this(delimiter)
        {
            _fileName = fileName;
        }

        public CsvLoader(StreamReader sr, char delimiter = ',')
            : this(delimiter)
        {
            _reader = sr;
        }

        private StreamReader GetReader()
        {
            if (_reader == null)
                _reader = new StreamReader(_fileName);

            return _reader;
        }

        private void CloseReaderIfNecessary()
        {
            if (_fileName == null)
            {
                _reader.Close();
                _reader = null;
            }
        }

        public Dictionary<string, List<string>> LoadData(Predicate<string> filter = null)
        {
            Dictionary<string, List<string>> ret = new Dictionary<string, List<string>>();

            if (filter == null)
                filter = ((s) => true);

            try
            {
                string line;
                string[] header = null;
                string[] columns;
                var sr = GetReader();

                while (!sr.EndOfStream)
                {
                    line = sr.ReadLine();
                    columns = line.Split(_delimiter);

                    if (header == null)
                    {
                        header = columns;

                        foreach (var item in header)
                        {
                            ret.Add(item, new List<string>());
                        }
                    }
                    else
                    {
                        int i;

                        /// make sure that the index will stay within the bounds of the header and the current row
                        for (i = 0; i < columns.Length && i < header.Length; i++)
                        {
                            ret[header[i]].Add(columns[i]);
                        }

                        /// in case some columns are missing in the current row, they will be filled up with NULL values
                        for (; i < header.Length; i++)
                        {
                            ret[header[i]].Add(null);
                        }
                    }
                }
            }
            finally
            {
                CloseReaderIfNecessary();
            }

            return ret;
        }
    }
}