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# C# Microsoft.Data.Analysis の DataFrame を CsvHelperを使用してCSVファイルとして出力する　2021-04-14

Microsoft.Data.Analysis の DataFrame には、一応簡易的なCSV出力機能が実装されているが、実際には使い物にならない為、CsvHelperで代用する。

## 環境
* Windows Server 2019
* Visual Studio 2019
* Microsoft.Data.Analysis 0.4.0
* CsvHelper 22.1.2
* .Net Framework 4.7.2

## CsvHelperでCSVファイルに出力する静的メソッド

        public static void WriteCsv(
            string csvPath,
            DataFrame df
            )
        {
            var fInfo = new FileInfo(csvPath);
            if (fInfo.Exists)
            {
                fInfo.Delete();
            }
            using (FileStream fs = fInfo.OpenWrite())
            {
                Encoding enc = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
                //Encoding enc = Encoding.Default;
                //Encoding enc = Encoding.UTF8;
                using (StreamWriter stream = new StreamWriter(fs, enc))
                {
                    //Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
                    //configuration.HasHeaderRecord = true;
                    CsvConfiguration configuration = new CsvConfiguration(
                        cultureInfo: CultureInfo.CurrentCulture,
                        encoding   : enc
                        );

                    using (CsvWriter writer = new CsvWriter(stream, configuration))
                    {
                        IReadOnlyList<string> columnNames = df
                            .Columns
                            .Select(n => n.Name)
                            .ToList();
                        foreach (var item in columnNames)
                        {
                            writer.WriteField(item);
                        }
                        writer.NextRecord();

                        foreach (DataFrameRow row in df.Rows)
                        {
                            //StringBuilder debug = new StringBuilder();
                            foreach (object item in row)
                            {
                                StringBuilder stringBuilder = new StringBuilder();
                                Type left = item?.GetType();
                                if (left == typeof(bool))
                                {
                                    stringBuilder.AppendFormat(configuration.CultureInfo, "{0}", item);
                                }
                                else if (left == typeof(float))
                                {
                                    stringBuilder.AppendFormat(configuration.CultureInfo, "{0:G9}", item);
                                }
                                else if (left == typeof(double))
                                {
                                    stringBuilder.AppendFormat(configuration.CultureInfo, "{0:G17}", item);
                                }
                                else if (left == typeof(decimal))
                                {
                                    stringBuilder.AppendFormat(configuration.CultureInfo, "{0:G31}", item);
                                }
                                else
                                {
                                    stringBuilder.Append(item);
                                }
                                //if (debug != null)
                                //{
                                //    debug.Append(stringBuilder.ToString()).Append(",");
                                //}
                                writer.WriteField(stringBuilder.ToString());
                                stringBuilder.Clear();
                            }
                            writer.NextRecord();

                            //Console.WriteLine(debug);
                        }

                        //stream.Flush();
                    }
                }
            }
        }

###### tags: `ML.NET` `Microsoft.Data.Analysis DataFrame` `C#` `CsvHelper`