DataTable to CSV extension methods. Export to CSV file, CSV string

CsvUtils.cs

public static class CSV
{
    public static string ToCSV(this DataTable table)
    {
        var columnHeaders = (from DataColumn x in table.Columns
                             select x.ColumnName).ToArray();
        StringBuilder builder = new StringBuilder(String.Join(",", columnHeaders));
        builder.Append("\n");

        foreach (DataRow row in table.Rows)
        {
            for (int i = 0; i < table.Columns.Count; i++)
            {
                builder.Append(row[i].ToString());
                builder.Append((i == table.Columns.Count - 1) ? "\n" : ",");
            }
        }

        return builder.ToString();
    }


    public static string ToExcelFormat(this DataTable table) 
    {
        var columnHeaders = (from DataColumn x in table.Columns
                             select x.ColumnName).ToArray();
        StringBuilder builder = new StringBuilder(String.Join(" ", columnHeaders));
        builder.Append("\n");

        foreach (DataRow row in table.Rows)
        {
            for (int i = 0; i < table.Columns.Count; i++)
            {
                builder.Append(row[i].ToString());
                builder.Append(i == table.Columns.Count - 1 ? "\n" : " ");
            }
        }

        return builder.ToString();
    }


    public static string ToDoubleQuotedCSV(this DataTable table)
    {
        var columnHeaders = (from DataColumn x in table.Columns
                             select x.ColumnName.GetDoubleQuotedString()).ToArray();
        StringBuilder builder = new StringBuilder(String.Join(",", columnHeaders));
        builder.Append("\n");
        
        foreach (DataRow row in table.Rows)
        {
            for (int i = 0; i < table.Columns.Count; i++)
            {
                builder.Append(row[i].ToString().GetDoubleQuotedString());
                builder.Append(i == table.Columns.Count - 1 ? "\n" : ",");
            }
        }

        return builder.ToString();
    }


    public static void ToCSVFile(this DataTable table, string filePath)
    {
        if (File.Exists(filePath))
        {
            throw new ArgumentException();
        }

        string writeData = table.ToCSV();
        writeData.WriteToFile(filePath);
    }


    public static void ToDoubleQuotedCSVFile(this DataTable table, string filePath)
    {
        if (File.Exists(filePath))
        {
            throw new ArgumentException();
        }

        string writeData = table.ToDoubleQuotedCSV();
        writeData.WriteToFile(filePath);
    }


    public static string ToCSVRow(this IEnumerable<string> strings)
    {
        return String.Join(",", strings);
    }


    public static string ToDoubleQuotedCSVRow(this IEnumerable<string> strings)
    {
        var strs = strings
                    .Select(s => s.GetDoubleQuotedString())
                    .ToArray();
        return String.Join(",", strs);
    }
}

Is GetDoubleQuotedString() a custom LINQ extension?