A simple, flexible CSV writer for C#. Modeled on the StreamWriter class.

CSVWriter.cs

/******************************************************************************
 * CSVWriter
 ******************************************************************************
 * Version 0.8
 * Author: Starbeamrainbowlabs <feedback at starbeamrainbowlabs dot com>
 *
 * An easy way to generate CSV files. Please allow this comment to remain
 * intact.
 ******************************************************************************
 * Changelog
 ******************************************************************************
 * 22nd December 2015:
 * 	 - Uploaded to GitHub gist.
 * 
 */

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

namespace CSVHandler
{
	/// <summary>
	/// A class that aids the writing of a (hopefully) fully valid CSV file.
	/// </summary>
	public class CSVWriter
	{
		/// <summary>
		/// The destination stream to which we will be writing the csv.
		/// </summary>
		private StreamWriter destination;

		/// <summary>
		/// Whether the underlying filestream is open or closed. True if the stream is open, false if closed.
		/// </summary>
		public bool IsOpen
		{
			get
			{
				return destination.BaseStream != null;
			}
		}

		/// <summary>
		/// Whether the headers have been writteen yet or not.
		/// </summary>
		private bool headersWritten = false;

		/// <summary>
		/// Whether the headers have been written.
		/// Note that attempting to write the headers again when they have already been written results in an exception been thrown.
		/// </summary>
		public bool HeadersWritten
		{
			get
			{
				return headersWritten;
			}
		}

		/// <summary>
		/// The number of fields that were specified when writing the headers.
		/// </summary>
		private int fieldCount;

		/// <summary>
		/// The number of fields declared when writing the headers. Records written to the CSV writer must have this number of fields, even if they are just empty strings.
		/// </summary>
		public int FieldCount
		{
			get
			{
				return fieldCount;
			}
		}

		/// <summary>
		/// The number of records that have been written to the output stream so far.
		/// </summary>
		private int recordsWritten = 0;

		/// <summary>
		/// The number of records written to the output stream so far. Note that this doesn't include the header line.
		/// </summary>
		public int RecordsWritten
		{
			get
			{
				return recordsWritten;
			}
		}

		/// <summary>
		/// Creates a new CSV writer that will write out to the given filename.
		/// </summary>
		/// <param name="filename">The filename to write the csv to.</param>
		public CSVWriter(string filename) : this(new StreamWriter(filename))
		{
		}

		/// <summary>
		/// Creates a new CSV writer that will write to the given StreamWriter.
		/// </summary>
		/// <param name="inDestination">The StreamWriter to write the csv to.</param>
		public CSVWriter(StreamWriter inDestination)
		{
			destination = inDestination;
		}

		/// <summary>
		/// Encapsulates a given string in quotes if it contains a comma.
		/// Rather useful when preparing cell data for writing.
		/// </summary>
		/// <param name="cellData">The string to check.</param>
		/// <returns>The properly escaped string.</returns>
		private string escapeCellData(string cellData)
		{
			if (cellData.Contains(","))
				return String.Format("\"{0}\"", cellData);
			else
				return cellData;
		}

		/// <summary>
		/// Writes the header contained in the given array to the file.
		/// </summary>
		/// <param name="inHeaders">The headers to write to the csv file.</param>
		public void WriteHeader(string[] inHeaders)
		{
			// Blow up if we try to write the headersmeor than once.
			if(headersWritten)
				throw new InvalidOperationException("You cannot write the CSV headers more than once.");

			// Save the number of headers for later
			fieldCount = inHeaders.Length;

			// Loop over each header and prepare it for writing. This encapsulates the header in quotes if it contians a comma.
			string[] preparedHeaders = new string[inHeaders.Length];
			for(int i = 0; i < inHeaders.Length; i++)
			{
				preparedHeaders[i] = escapeCellData(inHeaders[i]);
			}

			destination.WriteLine(String.Join(",", preparedHeaders));

			headersWritten = true;
		}

		/// <summary>
		/// Writes a new record to the csv file.
		/// Note that the number of fields in the data provided must the equal to that declared when writing the header.
		/// </summary>
		/// <param name="data">An array of records to write out.</param>
		public void WriteRecord(string[] data)
		{
			if(data.Length != FieldCount)
				throw new FormatException(String.Format("Data does not have the expected number of fields. Expected: {0}, Actual: {1}", data.Length, FieldCount));

			string[] preparedData = new string[data.Length];
			for(int i = 0; i < data.Length; i++)
			{
				preparedData[i] = escapeCellData(data[i]);
			}

			destination.WriteLine(String.Join(",", preparedData));

			recordsWritten++;
		}

		/// <summary>
		/// Closes the underlying StreamWriter.
		/// Don't attempt to write to the CSV writer after you've closed the file, as this will throw an exception!
		/// </summary>
		public void Close()
		{
			destination.Close();
		}
	}
}