File class Null safety

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 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

Implemented types
Implementers

Constructors

File(String path)
Creates a File object. [...]
factory
File.fromRawPath(Uint8List rawPath)
Creates a File object from a raw path. [...]
factory
File.fromUri(Uri uri)
Create a File object from a URI. [...]
factory

Properties

absolute File
A File with the absolute path of path. [...]
read-only, override
hashCode int
The hash code for this object. [...]
read-only, inherited
isAbsolute bool
Whether this object's path is absolute. [...]
read-only, inherited
parent Directory
The parent directory of this entity.
read-only, inherited
path String
Get the path of the file.
read-only, override
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited
uri Uri
A Uri representing the file system entity's location. [...]
read-only, inherited

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>
Deletes this FileSystemEntity. [...]
inherited
deleteSync({bool recursive: false}) → void
Synchronously deletes this FileSystemEntity. [...]
inherited
exists() Future<bool>
Checks whether the file system entity with this path exists. [...]
inherited
existsSync() bool
Synchronously checks whether the file system entity with this path exists. [...]
inherited
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
Invoked when a non-existent method or property is accessed. [...]
inherited
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>
Renames this file. [...]
override
renameSync(String newPath) File
Synchronously renames this file. [...]
override
Resolves the path of a file system object relative to the current working directory. [...]
inherited
resolveSymbolicLinksSync() String
Resolves the path of a file system object relative to the current working directory. [...]
inherited
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>
Calls the operating system's stat() function on path. [...]
inherited
statSync() FileStat
Synchronously calls the operating system's stat() function on path. [...]
inherited
toString() String
A string representation of this object. [...]
inherited
watch({int events: FileSystemEvent.all, bool recursive: false}) Stream<FileSystemEvent>
Start watching the FileSystemEntity for changes. [...]
inherited
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
The equality operator. [...]
inherited