ASP.NET Web API MediaTypeFormatter for converting a database command into CSV output

CsvDataFormatter.cs

#region Copyright (c) 2016 Atif Aziz. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

using System;
using System.Data.Common;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Net.Http.Formatting;
using System.Net.Http.Headers;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

public sealed class CsvDataFormatter : MediaTypeFormatter
{
    public CsvDataFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/csv"));
    }

    public override bool CanReadType(Type type) => false;

    public override bool CanWriteType(Type type) => type == typeof(Lazy<DbCommand>);

    public override async Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content,
        TransportContext transportContext, CancellationToken cancellationToken)
    {
        using (var command = ((Lazy<DbCommand>)value).Value)
        using (var reader = await command.ExecuteReaderAsync(cancellationToken))
        using (var writer = new StreamWriter(writeStream, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false)))
        {
            var ordinals = Enumerable.Range(0, reader.FieldCount).ToArray();

            var headers =
                from i in ordinals
                // ReSharper disable once AccessToDisposedClosure
                select Encode(reader.GetName(i));

            await writer.WriteLineAsync(string.Join(",", headers));

            while (await reader.ReadAsync(cancellationToken))
            {
                var fields =
                    from i in ordinals
                    // ReSharper disable once AccessToDisposedClosure
                    select reader.IsDBNull(i)
                         ? string.Empty
                           // ReSharper disable once AccessToDisposedClosure
                         : Encode(Convert.ToString(reader.GetValue(i), CultureInfo.InvariantCulture));

                await writer.WriteLineAsync(string.Join(",", fields));
            }
        }
    }

    static string Encode(string s)
    {
        const string quote = "\"";
        s = s.Replace(quote, quote + quote);
        return s.IndexOfAny(QuoteChars) >= 0 ? quote + s + quote : s;
    }

    static readonly char[] QuoteChars = { ',', '\r', '\n' };
}