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

using Microsoft.Office.Core;

using Excel = Microsoft.Office.Interop.Excel;

namespace excel_test
{
    class Program
    {
        
        // 获得字段的实际最大长度
        static int GetMaxLength(DataTable dt, string captionName)
        {
           DataColumn maxLengthColumn = new DataColumn();
           maxLengthColumn.ColumnName = "MaxLength";
           maxLengthColumn.Expression = String.Format("len(convert({0},‘System.String‘))", captionName);
           dt.Columns.Add(maxLengthColumn);
           object maxLength = dt.Compute("max(MaxLength)", "true");
           dt.Columns.Remove(maxLengthColumn);

           return Convert.ToInt32(maxLength);
        }

        static void convertExcelToTxt(string inputFile, string outputPath)
        {
            string newFileName = Path.GetFileNameWithoutExtension(inputFile) + ".txt";
            string newFile = outputPath + "\\" +newFileName;
            Console.WriteLine("Convert file[{0}] to [{1}]", inputFile, newFile);

            FileStream fs = new FileStream(newFile, FileMode.OpenOrCreate);
            StreamWriter sw = new StreamWriter(fs);

            var conn = new OleDbConnection();
            conn.ConnectionString = String.Format(@"Provider=Microsoft.ACE.OLEDB.12.0;" +
                @"Data Source={0}" +
                ";Extended Properties=\"Excel 12.0 Xml;HDR=No\"", inputFile);
            conn.Open();
            OleDbDataAdapter da = new OleDbDataAdapter("select * from [Sheet1$]", conn);
            var ds = new DataSet();
            da.Fill(ds);
            var tb1 = ds.Tables[0];
            
            foreach (DataRow row in tb1.Rows)
            {
                for (int j = 0; j < tb1.Columns.Count; ++j)
                {
                    DataColumn col = tb1.Columns[j];
                    string fmt = String.Format("{0}0,-{1}{2}", "{", GetMaxLength(tb1, col.Caption) + 2, "}");
                    sw.Write(fmt, row[j]);
                    Console.Write(fmt, row[j]);
                }
                sw.WriteLine();
                Console.WriteLine();
            }
            sw.Close();
        }

        static int Main(string[] args)
        {
            if(args.Length < 1)
            {
                Console.WriteLine("No input file arg.");
                return -1;
            }
            string inputFile = args[0];
            if (!File.Exists(inputFile))
            {
                Console.WriteLine("No input file exist, {0}", inputFile);
                return -1;
            }

            string fileName = Path.GetFileName(inputFile);
            string outputPath;
            if (args.Length > 1)
            {
                outputPath = args[1];
            }
            else
            {
                outputPath = Path.GetDirectoryName(inputFile);
            }
            if (!Directory.Exists(outputPath))
            {
                Console.WriteLine("Output directory not exist, {0}.", outputPath);
                return -1;
            }
            convertExcelToTxt(inputFile, outputPath);
            return 0;
        }
    }
}

参考：
1. 【C#读取Excel 2010】,汪宇杰
2. 【C#格式化字符串中转义大括号“{}”】, ^_^肥仔John
3. 【浅谈  DataSet   的用法】

用C#写一个Excel转Txt的小工具