Log data to CSV on HoloLens from a Unity project. Writes to Pictures folder instead of applicationDataPath so that files can be accessed from File Explorer (File Explorer -> HoloLens -> Pictures -> YourFolder). Doesn't require using Device portal.

CSVLogger.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using UnityEngine;
#if WINDOWS_UWP
using Windows.Storage;
#endif

namespace holoutils
{
    /// <summary>
    /// Component that Logs data to a CSV.
    /// Assumes header is fixed.
    /// Copy and paste this logger to create your own CSV logger.
    /// CSV Logger breaks data up into settions (starts when application starts) which are folders
    /// and instances which are files
    /// A session starts when the application starts, it ends when the session ends.
    /// 
    /// In Editor, writes to MyDocuments/SessionFolderRoot folder
    /// On Device, saves data in the Pictures/SessionFolderRoot
    /// 
    /// How to use:
    /// Find the csvlogger
    /// if it has not started a CSV, create one.
    /// every frame, log stuff
    /// Flush data regularly
    /// 
    /// **Important: Requires the PicturesLibrary capability!**
    /// </summary>
    public class CSVLogger : MonoBehaviour

    {
        #region Constants to modify
        private const string DataSuffix = "data";
        private const string CSVHeader = "Timestamp,SessionID,RecordingID," +
                                        "blah,blah,blah";
        private const string SessionFolderRoot = "CSVLogger";
        #endregion

        #region private members
        private string m_sessionPath;
        private string m_filePath;
        private string m_recordingId;
        private string m_sessionId;

        private StringBuilder m_csvData;
        #endregion
        #region public members
        public string RecordingInstance => m_recordingId;
        #endregion

        // Use this for initialization
        async void Start()
        {
            await MakeNewSession();
        }

        async Task MakeNewSession()
        {
            m_sessionId = DateTime.Now.ToString("yyyyMMdd_HHmmss");
            string rootPath = "";
#if WINDOWS_UWP
            StorageFolder sessionParentFolder = await KnownFolders.PicturesLibrary
                .CreateFolderAsync(SessionFolderRoot,
                CreationCollisionOption.OpenIfExists);
            rootPath = sessionParentFolder.Path;
#else
            rootPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), SessionFolderRoot);
            if (!Directory.Exists(rootPath)) Directory.CreateDirectory(rootPath);
#endif
            m_sessionPath = Path.Combine(rootPath, m_sessionId);
            Directory.CreateDirectory(m_sessionPath);
            Debug.Log("CSVLogger logging data to " + m_sessionPath);
        }

        public void StartNewCSV()
        {
            m_recordingId = DateTime.Now.ToString("yyyyMMdd_HHmmssfff");
            var filename = m_recordingId + "-" + DataSuffix + ".csv";
            m_filePath = Path.Combine(m_sessionPath, filename);
            if (m_csvData != null)
            {
                EndCSV();
            }
            m_csvData = new StringBuilder();
            m_csvData.AppendLine(CSVHeader);
        }


        public void EndCSV()
        {
            if (m_csvData == null)
            {
                return;
            }
            using (var csvWriter = new StreamWriter(m_filePath, true))
            {
                csvWriter.Write(m_csvData.ToString());
            }
            m_recordingId = null;
            m_csvData = null;
        }

        public void OnDestroy()
        {
            EndCSV();
        }

        public void AddRow(List<String> rowData)
        {
            AddRow(string.Join(",", rowData.ToArray()));
        }

        public void AddRow(string row)
        {
            m_csvData.AppendLine(row);
        }

        /// <summary>
        /// Writes all current data to current file
        /// </summary>
        public void FlushData()
        {
            using (var csvWriter = new StreamWriter(m_filePath, true))
            {
                csvWriter.Write(m_csvData.ToString());
            }
            m_csvData.Clear();
        }

        /// <summary>
        /// Returns a row populated with common start data like
        /// recording id, session id, timestamp
        /// </summary>
        /// <returns></returns>
        public List<String> RowWithStartData()
        {
            List<String> rowData = new List<String>();
            rowData.Add(Time.timeSinceLevelLoad.ToString("##.000"));
            rowData.Add(m_recordingId);
            rowData.Add(m_recordingId);
            return rowData;
        }

    }
}

Hello, thank you for the code it works well in unity but the data is quite delayed on the Hololens. At first, I didn't include FlushData() in the code, .csv file was created properly in the unity editor but it was not created on the Hololens. But right after I put FlushData(), I can see the file created in Hololens's picture folder but it makes the game and sensor data delayed more than I expected which is around 30sec from the normal 4sec (before adding flush). Do you have any suggestions or recommendations?

Thank you for the code. I have just one question, where should I call the StartNewCSV() and FlushData() functions? The code that you provided has a async void Start() function which does not allow me to have my basic void start(){}. I'm guessing I should use the FlushData() in the Update() function. right?

I think StartNewCSV() here is to create a CSV file with the unity editor. If you want to create a CSV file in the Hololens, you need to use the Windows.Storage for UWP. Referring to https://learn.microsoft.com/en-us/windows/uwp/files/quickstart-reading-and-writing-files

For more file-accessing codes for UWP, I found this helpful:
https://github.com/Microsoft/Windows-universal-samples/tree/main/Samples/FileAccess