File class abstract interface

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

  // Close the IOSink to free system resources.
  await 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.

Special 'nul' file

On Linux and Mac '/dev/null' and on Windows '\?\NUL' refer to a special file, such that all writes to it get consumed and disappear, and all reads produce empty output. Note that on Windows 'nul'(without '\?'-prefix) refers to a regular file named 'nul' in current directory.

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.