CsvActionResult.cs

using System;
using System.Data;
using System.IO;
using System.Text;
using System.Web;
using System.Web.Mvc;

public sealed class CsvActionResult : FileResult
{
    private readonly DataTable _dataTable;

    public CsvActionResult(DataTable dataTable)
        : base("text/csv")
    {
        _dataTable = dataTable;
    }

    protected override void WriteFile(HttpResponseBase response)
    {
        var outputStream = response.OutputStream;
        using (var memoryStream = new MemoryStream())
        {
            WriteDataTable(memoryStream);
            outputStream.Write(memoryStream.GetBuffer(), 0, (int)memoryStream.Length);
        }
    }

    private void WriteDataTable(Stream stream)
    {
        var streamWriter = new StreamWriter(stream, Encoding.Default);

        WriteHeaderLine(streamWriter);
        streamWriter.WriteLine();
        WriteDataLines(streamWriter);

        streamWriter.Flush();
    }

    private void WriteHeaderLine(StreamWriter streamWriter)
    {
        foreach (DataColumn dataColumn in _dataTable.Columns)
        {
            WriteValue(streamWriter, dataColumn.ColumnName);
        }
    }

    private void WriteDataLines(StreamWriter streamWriter)
    {
        foreach (DataRow dataRow in _dataTable.Rows)
        {
            foreach (DataColumn dataColumn in _dataTable.Columns)
            {
                WriteValue(streamWriter, dataRow[dataColumn.ColumnName].ToString());
            }
            streamWriter.WriteLine();
        }
    }


    private static void WriteValue(StreamWriter writer, String value)
    {
        writer.Write("\"");
        writer.Write(value.Replace("\"", "\"\""));
        writer.Write("\",");
    }
}