Writing a CSV File in .Net

Writing data into CSV file in C#

UPDATE

Back in my naïve days, I suggested doing this manually (it was a simple solution to a simple question), however due to this becoming more and more popular, I'd recommend using the library CsvHelper that does all the safety checks, etc.

CSV is way more complicated than what the question/answer suggests.

Original Answer

As you already have a loop, consider doing it like this:

//before your loop
    var csv = new StringBuilder();

//in your loop
    var first = reader[0].ToString();
    var second = image.ToString();
    //Suggestion made by KyleMit
    var newLine = string.Format("{0},{1}", first, second);
    csv.AppendLine(newLine);  

//after your loop
    File.WriteAllText(filePath, csv.ToString());

Or something to this effect.
My reasoning is: you won't be need to write to the file for every item, you will only be opening the stream once and then writing to it.

You can replace

File.WriteAllText(filePath, csv.ToString());

with

File.AppendAllText(filePath, csv.ToString());

if you want to keep previous versions of csv in the same file

C# 6

If you are using c# 6.0 then you can do the following

var newLine = $"{first},{second}"

EDIT

Here is a link to a question that explains what Environment.NewLine does.

How do I Create a CSV file in C#

You can write a csv file using streamwriter. Your file will be located in bin/Debug (if running debug mode and not stating otherwise).

 var filepath = "your_path.csv";
 using (StreamWriter writer = new StreamWriter(new FileStream(filepath,
 FileMode.Create, FileAccess.Write)))
 {
     writer.WriteLine("sep=,");
     writer.WriteLine("Hello, Goodbye");
 }

Writing a CSV file in .net

If there are any commas in your cell, surround the entire cell with double quotes, eg:

cell 1,cell 2,"This is one cell, even with a comma",cell4,etc

And if you want a literal double quote, do two of them, eg:

cell 1,cell 2,"This is my cell and it has ""quotes"" in it",cell 4,etc

As for dates, stick to ISO format, and you should be fine (eg yyyy-mm-dd hh:mm:ss)

Writing in an CSV file using C#

CSV is absolutely NOT a SIMPLE file format, it is a sufficiently elaborate format that is able to encompass almost any kind of data regardless of shape or size.

The CSV format is capable of dealing with optional parameters vs non optional, data with or without line breaks, and should be able to work with or without field escaping characters in any field without line breaks and is required to have field escaping characters in fields with line breaks.

You should not ever work with CSV files by hand, you should utilize FileHelpers to work with CSV files.

At this point I no longer use FileHelpers as my goto CSV parsing library I use CsvHelper from Josh Close.

Create a .csv file in C#

This the approach i normally take. Probably not the most efficient though.

        /// <summary>
    /// Generates the contents of the log file.
    /// </summary>
    /// <returns>The contents of the log file.</returns>
    internal string GenerateLogFile()
    {
        StringBuilder csvExport = new StringBuilder();
        csvExport.AppendLine(Resources.CSVHeader);

        foreach (DataRow row in this.logEntries.Rows)
        {
            csvExport.AppendLine(
                string.Format(
                "\"{0}\",\"{1}\",\"{2}\",\"{3}\",\"{4}\",\"{5}\",\"{6}\",\"{7}\",\"{8}\", \"{9}\"",
                row[ColumnNames.LogTime], row[ColumnNames.Field1], row[ColumnNames.Field2], row[ColumnNames.Field3], row[ColumnNames.Field4], row[ColumnNames.Field5], row[ColumnNames.Field6], row[ColumnNames.Field7], row[ColumnNames.Field8], row[ColumnNames.Field9]));
        }

        return csvExport.ToString();
    }

    /// <summary>
    /// Adds the CSV file to the response.
    /// </summary>
    /// <param name="csvExportContents">The contents of the CSV file.</param>
    internal void DisplayLogFile(string csvExportContents)
    {
        byte[] data = new ASCIIEncoding().GetBytes(csvExportContents);

        HttpContext.Current.Response.Clear();
        HttpContext.Current.Response.ContentType = "APPLICATION/OCTET-STREAM";
        HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename=Export.csv");
        HttpContext.Current.Response.OutputStream.Write(data, 0, data.Length);
        HttpContext.Current.Response.End();
    }

Write to CSV file and export it?

Rom, you're doing it wrong. You don't want to write files to disk so that IIS can serve them up. That adds security implications as well as increases complexity. All you really need to do is save the CSV directly to the response stream.

Here's the scenario: User wishes to download csv. User submits a form with details about the csv they want. You prepare the csv, then provide the user a URL to an aspx page which can be used to construct the csv file and write it to the response stream. The user clicks the link. The aspx page is blank; in the page codebehind you simply write the csv to the response stream and end it.

You can add the following to the (I believe this is correct) Load event:

string attachment = "attachment; filename=MyCsvLol.csv";
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.ClearHeaders();
HttpContext.Current.Response.ClearContent();
HttpContext.Current.Response.AddHeader("content-disposition", attachment);
HttpContext.Current.Response.ContentType = "text/csv";
HttpContext.Current.Response.AddHeader("Pragma", "public");

var sb = new StringBuilder();
foreach(var line in DataToExportToCSV)
  sb.AppendLine(TransformDataLineIntoCsv(line));

HttpContext.Current.Response.Write(sb.ToString());

writing to the response stream code ganked from here.

How to create multiple CSV file in ASP .Net

I am not sure what do you mean about follow problem.
#but not writing another CSV file#

in your code, obviously, you wrote same file in the loop; why do not you change it to

string fileName = "myfile"+listItem.Value+".csv";//change filename?
string filePath = Server.MapPath("~/file" + fileName);
using (StreamWriter sw = new StreamWriter(filePath, false))
{
     sw.Write(csv);
}