File class Null safety

A reference to a file on the file system.

A File instance is an object that holds a path on which operations can be performed. You can get the parent directory of the file using the getter 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 = new 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() {
  new 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. Listen to the stream for data and process as needed. You can use various transformers in succession to manipulate the data 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';

main() {
  final file = new File('file.txt');
  Stream<List<int>> inputStream = file.openRead();

  inputStream
    .transform(utf8.decoder)       // Decode bytes to UTF-8.
    .transform(new LineSplitter()) // Convert stream to individual lines.
    .listen((String line) {        // Process results.
        print('$line: ${line.length} bytes');
      },
      onDone: () { print('File is now closed.'); },
      onError: (e) { print(e.toString()); });
}

Write to a file

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

import 'dart:io';

void main() {
  final filename = 'file.txt';
  new File(filename).writeAsString('some content')
    .then((File file) {
      // 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 = new File('file.txt');
  var sink = file.openWrite();
  sink.write('FILE ACCESSED ${new DateTime.now()}\n');

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

The use of Futures

To avoid unintentional blocking of the program, several methods use a Future to return a value. For example, the length method, which gets the length of a file, returns a Future. Use then to register a callback function, which is called when the value is ready.

import 'dart:io';

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

  file.length().then((len) {
    print(len);
  });
}

In addition to length, the exists, lastModified, stat, and other methods, return Futures.

Other resources

• Dart by Example provides additional task-oriented code samples that show how to use various API from the Directory class and the related File class.

• I/O for Command-Line Apps a section from A Tour of the Dart Libraries covers files and directories.

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