NancyFx export Dapper dynamic query results to CSV using CsvHelper

DapperHelper.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Web;

namespace nancyfx_dapper_servicestack_text_csvhelper
{
    public class DapperHelper
    {
        public static string DapperQueryToCsvString(IEnumerable<dynamic> dapperQueryResults)
        {
            if (isEmpty(dapperQueryResults)) return string.Empty;

            // results not empty


            StringWriter sw = new StringWriter(); // defaults to System.Text.UnicodeEncoding

            DapperQueryToTextWriter(dapperQueryResults, (TextWriter)sw);
            return sw.ToString();
        }

        public static Stream DapperQueryToCsvStream(IEnumerable<dynamic> dapperQueryResults)
        {
            if (isEmpty(dapperQueryResults)) return new MemoryStream();

            // results not empty

            MemoryStream ms = new MemoryStream();
            StreamWriter sw = new StreamWriter(ms, Encoding.UTF8); 

            DapperQueryToTextWriter(dapperQueryResults, (TextWriter)sw);
            sw.Flush();
            return ms;
        }

        private static void DapperQueryToTextWriter(IEnumerable<dynamic> dapperQueryResults, TextWriter tw)
        {
            var csv = new CsvHelper.CsvWriter(tw);
            var dapperRows = dapperQueryResults.Cast<IDictionary<string, object>>().ToList();
            bool headerWritten = false;
            foreach (IDictionary<string, object> row in dapperRows)
            {
                if (!headerWritten)
                {
                    foreach (KeyValuePair<string, object> item in row)
                    {
                        csv.WriteField(item.Key);
                    }
                    csv.NextRecord();
                    headerWritten = true;
                }
                foreach (KeyValuePair<string, object> item in row)
                {
                    csv.WriteField(item.Value);
                }
                csv.NextRecord();
            }
            csv.Flush();
        }

        public static bool isEmpty(IEnumerable<dynamic> queryResults)
        {
            if (queryResults != null && queryResults.GetEnumerator().MoveNext())
            {
                return false;
            }
            else
            {
                return true;
            }
        }

        //// change StringWriter encoding to UTF-8 https://stackoverflow.com/a/1564727
        //public sealed class Utf8StringWriter : StringWriter
        //{
        //    public override Encoding Encoding => Encoding.UTF8;
        //}
    }
}

packages.json

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="CsvHelper" version="4.0.2" targetFramework="net461" />
  <package id="Dapper" version="1.50.2" targetFramework="net461" />
  <package id="Microsoft.CodeDom.Providers.DotNetCompilerPlatform" version="1.0.7" targetFramework="net461" />
  <package id="Microsoft.Net.Compilers" version="2.1.0" targetFramework="net461" developmentDependency="true" />
  <package id="Nancy" version="1.4.1" targetFramework="net461" />
  <package id="Nancy.Hosting.Aspnet" version="1.4.1" targetFramework="net461" />
  <package id="Nancy.Serialization.ServiceStack" version="1.4.1" targetFramework="net461" />
  <package id="ServiceStack.Text" version="3.9.11" targetFramework="net461" />
</packages>

TestModule.cs

using Dapper;
using Nancy;
using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.IO;
using System.Linq;
using System.Web;

namespace nancyfx_dapper_servicestack_text_csvhelper
{
    public class TestModule : NancyModule
    {
        public TestModule(SqlConnection conn) : base("/api")
        {
            Get["Suppliers"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                    return Response.AsJson(results);
                //http://localhost:50416/api/Suppliers resulting json has special characters encoded correctly
            };

            Get["SuppliersCsv"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                    return Response.AsText(DapperHelper.DapperQueryToCsvString(results));
                //http://localhost:50416/api/SuppliersCsv special characters are displayed incorrectly in browser
            };

            Get["SuppliersCsv1"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                    return Response.AsText(DapperHelper.DapperQueryToCsvString(results), "text/html");
                //http://localhost:50416/api/SuppliersCsv1 special characters are displayed incorrectly in browser
            };

            Get["SuppliersCsv2"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                    return Response.AsText(DapperHelper.DapperQueryToCsvString(results), "text/html;charset=utf-8");
                //http://localhost:50416/api/SuppliersCsv2 special characters are displayed correctly in browser
            };

            Get["SuppliersCsv3"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                    return Response.AsText(DapperHelper.DapperQueryToCsvString(results), "text/html;charset=utf-8")
                        .WithHeader("Content-disposition", "attachment;filename=Export3.csv");
                //http://localhost:50416/api/SuppliersCsv3 browser downloads Export.csv attachment. Excel displays special characters incorrectly.
            };

            Get["SuppliersCsv4"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                {
                    Stream stream = DapperHelper.DapperQueryToCsvStream(results);
                    stream.Position = 0;
                    return Response.FromStream(stream, "text/html;charset=utf-8");
                    //http://localhost:50416/api/SuppliersCsv4 browser displays special characters correctly.
                }
            };

            Get["SuppliersCsv5"] = _ =>
            {
                var results = conn.Query("select top 15 * from Suppliers");
                if (DapperHelper.isEmpty(results))
                    return HttpStatusCode.NotFound;
                else
                {
                    Stream stream = DapperHelper.DapperQueryToCsvStream(results);
                    stream.Position = 0;
                    return Response.FromStream(stream, "text/html;charset=utf-8")
                        .WithHeader("Content-disposition", "attachment;filename=Export5.csv");
                    //http://localhost:50416/api/SuppliersCsv5 browser download Export.csv attachement. Excel displays special characters correctly since file has the UTF8 BOM: EF BB BF
                    //https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/
                    //https://superuser.com/questions/1204233/excel-save-behaviour-of-csv-file-with-utf8-encoding-vs-utf8-bom-encoding

                }
            };
        }
    }
}