dart:io

File class

A reference to a file on the file system.

A File holds a path on which operations can be performed. You can get the parent directory of the file using parent, a property inherited from FileSystemEntity.

Create a new File object with a pathname to access the specified file on the file system from your program.

var myFile = File('file.txt');

The File class contains methods for manipulating files and their contents. Using methods in this class, you can open and close files, read to and write from them, create and delete them, and check for their existence.

When reading or writing a file, you can use streams (with openRead), random access operations (with open), or convenience methods such as readAsString,

Most methods in this class occur in synchronous and asynchronous pairs, for example, readAsString and readAsStringSync. Unless you have a specific reason for using the synchronous version of a method, prefer the asynchronous version to avoid blocking your program.

If path is a link

If path is a symbolic link, rather than a file, then the methods of File operate on the ultimate target of the link, except for delete and deleteSync, which operate on the link.

Read from a file

The following code sample reads the entire contents from a file as a string using the asynchronous readAsString method:

import 'dart:async';
import 'dart:io';

void main() {
  File('file.txt').readAsString().then((String contents) {
    print(contents);
  });
}

A more flexible and useful way to read a file is with a Stream. Open the file with openRead, which returns a stream that provides the data in the file as chunks of bytes. Read the stream to process the file contents when available. You can use various transformers in succession to manipulate the file content into the required format, or to prepare it for output.

You might want to use a stream to read large files, to manipulate the data with transformers, or for compatibility with another API, such as WebSockets.

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

void main() async {
  final file = File('file.txt');
  Stream<String> lines = file.openRead()
    .transform(utf8.decoder)       // Decode bytes to UTF-8.
    .transform(LineSplitter());    // Convert stream to individual lines.
  try {
    await for (var line in lines) {
      print('$line: ${line.length} characters');
    }
    print('File is now closed.');
  } catch (e) {
    print('Error: $e');
  }
}

Write to a file

To write a string to a file, use the writeAsString method:

import 'dart:io';

void main() async {
  final filename = 'file.txt';
  var file = await File(filename).writeAsString('some content');
  // Do something with the file.
}

You can also write to a file using a Stream. Open the file with openWrite, which returns an IOSink to which you can write data. Be sure to close the sink with the IOSink.close method.

import 'dart:io';

void main() {
  var file = File('file.txt');
  var sink = file.openWrite();
  sink.write('FILE ACCESSED ${DateTime.now()}\n');

  // Close the IOSink to free system resources.
  sink.close();
}

The use of asynchronous methods

To avoid unintentional blocking of the program, several methods are asynchronous and return a Future. For example, the length method, which gets the length of a file, returns a Future. Wait for the future to get the result when it's ready.

import 'dart:io';

void main() async {
  final file = File('file.txt');

  var length = await file.length();
  print(length);
}

In addition to length, the exists, lastModified, stat, and other methods, are asynchronous.

Other resources

• The Files and directories section of the library tour.

• Write Command-Line Apps, a tutorial about writing command-line apps, includes information about files and directories.

Implemented types

• FileSystemEntity

Constructors

File(String path)

factory

Creates a File object. [...]

File.fromRawPath(Uint8List rawPath)

factory

Creates a File object from a raw path. [...]

File.fromUri(Uri uri)

factory

Create a File object from a URI. [...]

Properties

absolute → File

read-only, override

A File with the absolute path of path. [...]

hashCode → int

read-only, inherited

The hash code for this object. [...]

isAbsolute → bool

read-only, inherited

Whether this object's path is absolute. [...]

parent → Directory

read-only, inherited

The parent directory of this entity.

path → String

read-only, override

Get the path of the file.

runtimeType → Type

read-only, inherited

A representation of the runtime type of the object.

uri → Uri

read-only, inherited

A Uri representing the file system entity's location. [...]

Methods

copy(String newPath) → Future<File>

Copies this file. [...]

copySync(String newPath) → File

Synchronously copies this file. [...]

create({bool recursive = false}) → Future<File>

Creates the file. [...]

createSync({bool recursive = false}) → void

Synchronously creates the file. [...]

delete({bool recursive = false}) → Future<FileSystemEntity>

inherited

Deletes this FileSystemEntity. [...]

deleteSync({bool recursive = false}) → void

inherited

Synchronously deletes this FileSystemEntity. [...]

exists() → Future<bool>

inherited

Checks whether the file system entity with this path exists. [...]

existsSync() → bool

inherited

Synchronously checks whether the file system entity with this path exists. [...]

lastAccessed() → Future<DateTime>

The last-accessed time of the file. [...]

lastAccessedSync() → DateTime

The last-accessed time of the file. [...]

lastModified() → Future<DateTime>

Get the last-modified time of the file. [...]

lastModifiedSync() → DateTime

Get the last-modified time of the file. [...]

length() → Future<int>

The length of the file. [...]

lengthSync() → int

The length of the file provided synchronously. [...]

noSuchMethod(Invocation invocation) → dynamic

inherited

Invoked when a non-existent method or property is accessed. [...]

open({FileMode mode = FileMode.read}) → Future<RandomAccessFile>

Opens the file for random access operations. [...]

openRead([int? start, int? end]) → Stream<List<int>>

Creates a new independent Stream for the contents of this file. [...]

openSync({FileMode mode = FileMode.read}) → RandomAccessFile

Synchronously opens the file for random access operations. [...]

openWrite({FileMode mode = FileMode.write, Encoding encoding = utf8}) → IOSink

Creates a new independent IOSink for the file. [...]

readAsBytes() → Future<Uint8List>

Reads the entire file contents as a list of bytes. [...]

readAsBytesSync() → Uint8List

Synchronously reads the entire file contents as a list of bytes. [...]

readAsLines({Encoding encoding = utf8}) → Future<List<String>>

Reads the entire file contents as lines of text using the given Encoding. [...]

readAsLinesSync({Encoding encoding = utf8}) → List<String>

Synchronously reads the entire file contents as lines of text using the given Encoding. [...]

readAsString({Encoding encoding = utf8}) → Future<String>

Reads the entire file contents as a string using the given Encoding. [...]

readAsStringSync({Encoding encoding = utf8}) → String

Synchronously reads the entire file contents as a string using the given Encoding. [...]

rename(String newPath) → Future<File>

override

Renames this file. [...]

renameSync(String newPath) → File

override

Synchronously renames this file. [...]

resolveSymbolicLinks() → Future<String>

inherited

Resolves the path of a file system object relative to the current working directory. [...]

resolveSymbolicLinksSync() → String

inherited

Resolves the path of a file system object relative to the current working directory. [...]

setLastAccessed(DateTime time) → Future

Modifies the time the file was last accessed. [...]

setLastAccessedSync(DateTime time) → void

Synchronously modifies the time the file was last accessed. [...]

setLastModified(DateTime time) → Future

Modifies the time the file was last modified. [...]

setLastModifiedSync(DateTime time) → void

Synchronously modifies the time the file was last modified. [...]

stat() → Future<FileStat>

inherited

Calls the operating system's stat() function on path. [...]

statSync() → FileStat

inherited

Synchronously calls the operating system's stat() function on path. [...]

toString() → String

inherited

A string representation of this object. [...]

watch({int events = FileSystemEvent.all, bool recursive = false}) → Stream<FileSystemEvent>

inherited

Start watching the FileSystemEntity for changes. [...]

writeAsBytes(List<int> bytes, {FileMode mode = FileMode.write, bool flush = false}) → Future<File>

Writes a list of bytes to a file. [...]

writeAsBytesSync(List<int> bytes, {FileMode mode = FileMode.write, bool flush = false}) → void

Synchronously writes a list of bytes to a file. [...]

writeAsString(String contents, {FileMode mode = FileMode.write, Encoding encoding = utf8, bool flush = false}) → Future<File>

Writes a string to a file. [...]

writeAsStringSync(String contents, {FileMode mode = FileMode.write, Encoding encoding = utf8, bool flush = false}) → void

Synchronously writes a string to a file. [...]

Operators

operator ==(Object other) → bool

inherited

The equality operator. [...]