Mail Parser in C# (in development)

Program.cs

/*
 * Copyright (c) 2017 Yuki Ono
 * Licensed under the MIT License.
 */

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace mail_parser_cs {
    class Program {
        static string[] unfold_header(string[] lines) {
            if (Char.IsWhiteSpace(lines.First().First()))
                throw new ArgumentException();

            var new_lines = new List<string>();

            foreach (var i in Enumerable.Range(0, lines.Length)) {
                if (lines[i] == "")
                    break;

                if (Char.IsWhiteSpace(lines[i].First())) {
                    var temp = new_lines[i - 1];
                    new_lines[i - 1] = temp + " " + lines[i].TrimStart();
                }
                else
                    new_lines.Add(lines[i]);
            }

            return new_lines.ToArray();
        }

        static (string, string) split_key_value(string line) {
            var kv = line.Split(":".ToCharArray(), 2);
            return (kv[0], kv[1]);
        }

        static Dictionary<string, List<string>> header_to_dict(string[] lines) {
            var dict = new Dictionary<string, List<string>>();

            foreach (var l in lines) {
                var (key, value) = split_key_value(l);

                var trimed_key = key.Trim();
                var trimed_value = value.Trim();

                if (dict.ContainsKey(trimed_key))
                    dict[trimed_key].Add(trimed_value);
                else {
                    var list = new List<string> { trimed_value };
                    dict.Add(trimed_key, list);
                }
            }

            return dict;
        }

        static (string[], string[]) split_header_body(string[] lines) {
            var sep_idx = Array.IndexOf(lines, "");
            Console.WriteLine("sepIdx: " + sep_idx);

            var header = unfold_header(lines.Take(sep_idx).ToArray());
            var body = lines.Skip(sep_idx).ToArray();

            return (header, body);
        }

        static void proc_body(string[] body) {
            // preamble
            // boundary
            // epilogue
        }

        static void proc_mail(string file) {
            var lines = File.ReadAllLines(file);

            var (header, body) = split_header_body(lines);
            var header_dict = header_to_dict(header);

            foreach (var kv in header_dict)
                Console.WriteLine("key: {0} value: {1}", kv.Key, string.Join(",", kv.Value));

            Console.WriteLine("body:\n" + string.Join("\n", body));

            proc_body(body);
        }

        static int Main(string[] args) {
            if (args.Length == 0) {
                Console.WriteLine("args.Length == 0");
                return 1;
            }

            var file = args[0];

            if (!File.Exists(file)) {
                Console.WriteLine("!File.Exists");
                return 1;
            }

            proc_mail(file);

            Console.Read();
            return 0;
        }
    }
}