Dart File Class
last modified April 4, 2025
The File class in Dart provides methods for working with files.
It allows reading, writing, and manipulating file contents and metadata.
File operations are asynchronous by default, using Dart's Future API.
The class is part of Dart's dart:io library for server-side apps.
Basic Definition
File represents a reference to a file in the file system.
It provides both synchronous and asynchronous methods for file operations.
Key features include reading/writing text/binary data, file metadata access, and file manipulation like copying or deleting. It works with both relative and absolute paths.
Reading a Text File
This example shows how to read the contents of a text file asynchronously.
import 'dart:io';
void main() async {
var file = File('example.txt');
try {
var contents = await file.readAsString();
print(contents);
} catch (e) {
print('Error reading file: $e');
}
}
We create a File object pointing to 'example.txt', then read its contents. The readAsString() method returns a Future that completes with the file's text.
$ dart main.dart Hello from example.txt
Writing to a File
This example demonstrates writing text to a file asynchronously.
import 'dart:io';
void main() async {
var file = File('output.txt');
try {
await file.writeAsString('Dart File example\n', mode: FileMode.append);
print('File written successfully');
} catch (e) {
print('Error writing file: $e');
}
}
We open 'output.txt' in append mode and add a line of text. FileMode.append ensures new content is added to the end rather than overwriting existing data.
$ dart main.dart File written successfully
Reading File Metadata
This example shows how to get information about a file.
import 'dart:io';
void main() async {
var file = File('example.txt');
try {
var stat = await file.stat();
print('File size: ${stat.size} bytes');
print('Modified: ${stat.modified}');
print('Accessed: ${stat.accessed}');
} catch (e) {
print('Error getting file stats: $e');
}
}
The stat() method returns a FileStat object containing file metadata. This includes size, modification time, and access time information.
$ dart main.dart File size: 24 bytes Modified: 2025-04-04 10:30:45.000 Accessed: 2025-04-04 10:35:12.000
Copying and Deleting Files
This example demonstrates file manipulation operations.
import 'dart:io';
void main() async {
var source = File('source.txt');
var dest = File('backup.txt');
try {
await source.copy(dest.path);
print('File copied successfully');
await dest.delete();
print('Backup file deleted');
} catch (e) {
print('Error manipulating files: $e');
}
}
We copy source.txt to backup.txt, then delete the backup file. Both operations are asynchronous and return Futures that complete when done.
$ dart main.dart File copied successfully Backup file deleted
Reading Binary Data
This example shows how to read a file as bytes.
import 'dart:io';
void main() async {
var file = File('image.png');
try {
var bytes = await file.readAsBytes();
print('Read ${bytes.length} bytes');
print('First byte: ${bytes[0]}');
} catch (e) {
print('Error reading binary file: $e');
}
}
The readAsBytes() method reads the entire file into a Uint8List. This is useful for binary files like images where text reading isn't appropriate.
$ dart main.dart Read 1024 bytes First byte: 137
Best Practices
- Error handling: Always handle potential IO errors
- Async operations: Prefer async methods for better performance
- Resource cleanup: Close streams when done with them
- Path handling: Use path.join() for cross-platform paths
Source
This tutorial covered Dart's File class with practical examples showing common file operations including reading, writing, and file manipulation.
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