How to create and export csv file in flutter?

You can copy paste run full code below
The following working demo write file to Downloads directory and view file with CSV Viwer
Step 1: Add <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> to AndroidManifest.xml
Step 2: request permission with package permission_handler
Step 3: get Downloads directory with package ext_storage
Step 4: use package csv's ListToCsvConverter()

code snippet

Map<Permission, PermissionStatus> statuses = await [
  Permission.storage,
].request();

List<dynamic> associateList = [
  {"number": 1, "lat": "14.97534313396318", "lon": "101.22998536005622"},
  {"number": 2, "lat": "14.97534313396318", "lon": "101.22998536005622"},
  {"number": 3, "lat": "14.97534313396318", "lon": "101.22998536005622"},
  {"number": 4, "lat": "14.97534313396318", "lon": "101.22998536005622"}
];

List<List<dynamic>> rows = [];

List<dynamic> row = [];
row.add("number");
row.add("latitude");
row.add("longitude");
rows.add(row);
for (int i = 0; i < associateList.length; i++) {
  List<dynamic> row = [];
  row.add(associateList[i]["number"] - 1);
  row.add(associateList[i]["lat"]);
  row.add(associateList[i]["lon"]);
  rows.add(row);
}

String csv = const ListToCsvConverter().convert(rows);

String dir = await ExtStorage.getExternalStoragePublicDirectory(
    ExtStorage.DIRECTORY_DOWNLOADS);
print("dir $dir");
String file = "$dir";

File f = File(file + "/filename.csv");

f.writeAsString(csv);

working demo

full code

import 'package:csv/csv.dart';
import 'package:flutter/material.dart';
import 'package:ext_storage/ext_storage.dart';
import 'dart:io';
import 'package:permission_handler/permission_handler.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _generateCsvFile() async {
    Map<Permission, PermissionStatus> statuses = await [
      Permission.storage,
    ].request();

    List<dynamic> associateList = [
      {"number": 1, "lat": "14.97534313396318", "lon": "101.22998536005622"},
      {"number": 2, "lat": "14.97534313396318", "lon": "101.22998536005622"},
      {"number": 3, "lat": "14.97534313396318", "lon": "101.22998536005622"},
      {"number": 4, "lat": "14.97534313396318", "lon": "101.22998536005622"}
    ];

    List<List<dynamic>> rows = [];

    List<dynamic> row = [];
    row.add("number");
    row.add("latitude");
    row.add("longitude");
    rows.add(row);
    for (int i = 0; i < associateList.length; i++) {
      List<dynamic> row = [];
      row.add(associateList[i]["number"] - 1);
      row.add(associateList[i]["lat"]);
      row.add(associateList[i]["lon"]);
      rows.add(row);
    }

    String csv = const ListToCsvConverter().convert(rows);

    String dir = await ExtStorage.getExternalStoragePublicDirectory(
        ExtStorage.DIRECTORY_DOWNLOADS);
    print("dir $dir");
    String file = "$dir";

    File f = File(file + "/filename.csv");

    f.writeAsString(csv);

    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _generateCsvFile,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

To export the list to a CSV file in Flutter, you can use the csv package like this:

import 'dart:io';
import 'package:csv/csv.dart';

void main() {
  List<Map<String, dynamic>> associateList = [
    {"number": 1, "lat": "14.97534313396318", "lon": "101.22998536005622"},
    {"number": 2, "lat": "14.97534313396318", "lon": "101.22998536005622"},
    {"number": 3, "lat": "14.97534313396318", "lon": "101.22998536005622"},
    {"number": 4, "lat": "14.97534313396318", "lon": "101.22998536005622"}
  ];

  exportCSV(associateList);
}

void exportCSV(List<Map<String, dynamic>> list) {
  List<List<dynamic>> rows = [];
  rows.add(["number", "lat", "lon"]);

  for (var map in list) {
    rows.add([map["number"], map["lat"], map["lon"]]);
  }

  String csv = const ListToCsvConverter().convert(rows);
  String dir = (await getExternalStorageDirectory()).path;
  String filePath = "$dir/list.csv";

  File file = File(filePath);
  await file.writeAsString(csv);

  print("File exported successfully!");
}

I have create a CSV file Using A to D width and Hight is 1 to 50

add tree library in Yaml file

 syncfusion_flutter_xlsio: ^20.1.60-beta
  path_provider: ^2.0.11
  open_file: ^3.2.1

if Column, Row in override issue

import 'package:syncfusion_flutter_xlsio/xlsio.dart' hide Column,  Row;

you can check only creatExcel()

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:syncfusion_flutter_xlsio/xlsio.dart';
import 'package:path_provider/path_provider.dart';
import 'package:open_file/open_file.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: ElevatedButton(
          onPressed: () {
            creatExcel();
          },
          child: Text("creat Excel"),
        ),
      ),
    );
  }

  creatExcel() async {
    Workbook workbook = Workbook();
// Accessing worksheet via index.
    final Worksheet sheet = workbook.worksheets[0];

    for (var i = 1; i < 50; i++) {
      sheet.getRangeByName('A$i').setText('Hello World!');
      sheet.getRangeByName('B$i').setText('Hello World!');
      sheet.getRangeByName('C$i').setText('Hello World!');
      sheet.getRangeByName('D$i').setText('Hello World!');
    }

    final List<int> bytes = workbook.saveAsStream();
    workbook.dispose();
    final String path = (await getApplicationDocumentsDirectory()).path;
    final String filename = '$path/output.xlsx';
    final File file = File(filename);

    await file.writeAsBytes(bytes, flush: true);
    OpenFile.open(filename);
  }
}

This package is well described and very easy to use for generating csv/excel files... pub.dev/packages/to_csv. This also minimize most of the code given in the answer by @chunhunghan

Code

import 'package:to_csv/to_csv.dart' as exportCSV;


List<String> header = [];
header.add('No.');
header.add('User Name');
header.add('Mobile');
header.add('ID Number');

List<List<String>> listOfLists = [];
List<String> data1 = ['1','Bilal Saeed','1374934','912839812']; 
List<String> data2 = ['2','Ahmar','21341234','192834821'];

listOfLists.add(data1);
listOfLists.add(data2);

exportCSV.myCSV(header, listOfLists);