A Tour of the dart:io Library

The dart:io library provides APIs to deal with files, directories, processes, sockets, WebSockets, and HTTP clients and servers.

Important: Only Flutter mobile apps, command-line scripts, and servers can import and use dart:io, not web apps.

In general, the dart:io library implements and promotes an asynchronous API. Synchronous methods can easily block an application, making it difficult to scale. Therefore, most operations return results via Future or Stream objects, a pattern common with modern server platforms such as Node.js.

The few synchronous methods in the dart:io library are clearly marked with a Sync suffix on the method name. Synchronous methods aren’t covered here.

To use the dart:io library you must import it:

import 'dart:io';

Files and directories

The I/O library enables command-line apps to read and write files and browse directories. You have two choices for reading the contents of a file: all at once, or streaming. Reading a file all at once requires enough memory to store all the contents of the file. If the file is very large or you want to process it while reading it, you should use a Stream, as described in Streaming file contents.

Reading a file as text

When reading a text file encoded using UTF-8, you can read the entire file contents with readAsString(). When the individual lines are important, you can use readAsLines(). In both cases, a Future object is returned that provides the contents of the file as one or more strings.

Future main() async {
  var config = File('config.txt');
  var contents;

  // Put the whole file in a single string.
  contents = await config.readAsString();
  print('The file is ${contents.length} characters long.');

  // Put each line of the file into its own string.
  contents = await config.readAsLines();
  print('The file is ${contents.length} lines long.');

Reading a file as binary

The following code reads an entire file as bytes into a list of ints. The call to readAsBytes() returns a Future, which provides the result when it’s available.

Future main() async {
  var config = File('config.txt');

  var contents = await config.readAsBytes();
  print('The file is ${contents.length} bytes long.');

Handling errors

To capture errors so they don’t result in uncaught exceptions, you can register a catchError handler on the Future, or (in an async function) use try-catch:

Future main() async {
  var config = File('config.txt');
  try {
    var contents = await config.readAsString();
  } catch (e) {

Streaming file contents

Use a Stream to read a file, a little at a time. You can use either the Stream API or await for, part of Dart’s asynchrony support.

import 'dart:io';
import 'dart:convert';

Future main() async {
  var config = File('config.txt');
  Stream<List<int>> inputStream = config.openRead();

  var lines = inputStream
  try {
    await for (var line in lines) {
      print('Got ${line.length} characters from stream');
    print('file is now closed');
  } catch (e) {

Writing file contents

You can use an IOSink to write data to a file. Use the File openWrite() method to get an IOSink that you can write to. The default mode, FileMode.write, completely overwrites existing data in the file.

var logFile = File('log.txt');
var sink = logFile.openWrite();
sink.write('FILE ACCESSED ${DateTime.now()}\n');
await sink.flush();
await sink.close();

To add to the end of the file, use the optional mode parameter to specify FileMode.append:

var sink = logFile.openWrite(mode: FileMode.append);

To write binary data, use add(List<int> data).

Listing files in a directory

Finding all files and subdirectories for a directory is an asynchronous operation. The list() method returns a Stream that emits an object when a file or directory is encountered.

Future main() async {
  var dir = Directory('tmp');

  try {
    var dirList = dir.list();
    await for (FileSystemEntity f in dirList) {
      if (f is File) {
        print('Found file ${f.path}');
      } else if (f is Directory) {
        print('Found dir ${f.path}');
  } catch (e) {

Other common functionality

The File and Directory classes contain other functionality, including but not limited to:

  • Creating a file or directory: create() in File and Directory
  • Deleting a file or directory: delete() in File and Directory
  • Getting the length of a file: length() in File
  • Getting random access to a file: open() in File

Refer to the API docs for File and Directory for a full list of methods.

HTTP clients and servers

The dart:io library provides classes that command-line apps can use for accessing HTTP resources, as well as running HTTP servers.

HTTP server

The HttpServer class provides the low-level functionality for building web servers. You can match request handlers, set headers, stream data, and more.

The following sample web server returns simple text information. This server listens on port 8888 and address (localhost), responding to requests for the path /dart. For any other path, the response is status code 404 (page not found).

Future main() async {
  final requests = await HttpServer.bind('localhost', 8888);
  await for (var request in requests) {

void processRequest(HttpRequest request) {
  print('Got request for ${request.uri.path}');
  final response = request.response;
  if (request.uri.path == '/dart') {
      ..headers.contentType = ContentType(
      ..write('Hello from the server');
  } else {
    response.statusCode = HttpStatus.notFound;

HTTP client

The HttpClient class helps you connect to HTTP resources from your Dart command-line or server-side application. You can set headers, use HTTP methods, and read and write data. The HttpClient class does not work in browser-based apps. When programming in the browser, use the dart:html HttpRequest class. Here’s an example of using HttpClient:

Future main() async {
  var url = Uri.parse('http://localhost:8888/dart');
  var httpClient = HttpClient();
  var request = await httpClient.getUrl(url);
  var response = await request.close();
  var data = await response.transform(utf8.decoder).toList();
  print('Response ${response.statusCode}: $data');

More information

This page showed how to use the major features of the dart:io library. Besides the APIs discussed in this section, the dart:io library also provides APIs for processes, sockets, and web sockets. For more information about server-side and command-line app development, see the server-side Dart overview.

For information on other dart:* libraries, see the library tour.