Dart FileMode
last modified April 4, 2025
The FileMode class in Dart defines constants for file opening modes.
It's used with file operations to specify how a file should be opened.
FileMode is essential for controlling file access permissions and behavior.
It's part of Dart's dart:io library for file system operations.
Basic Definition
FileMode is an enum-like class with constants for file operations.
It determines whether to read, write, append, or create files.
Key modes include read, write, append, and writeOnly. These control file access and whether to create or truncate existing files.
Reading a File
This example shows how to open a file in read-only mode using FileMode.read.
import 'dart:io';
void main() async {
var file = File('example.txt');
try {
var contents = await file.open(mode: FileMode.read);
var stream = contents.transform(utf8.decoder);
await for (var line in stream) {
print(line);
}
await contents.close();
} catch (e) {
print('Error: $e');
}
}
We open a file in read mode and read its contents as a stream. FileMode.read ensures we only have read access to the file. The file must exist for this mode.
$ dart main.dart Hello, Dart! This is a test file.
Writing to a File
This example demonstrates writing to a file using FileMode.write.
import 'dart:io';
void main() async {
var file = File('output.txt');
try {
var sink = file.openWrite(mode: FileMode.write);
sink.write('First line\n');
sink.write('Second line\n');
await sink.close();
print('File written successfully');
} catch (e) {
print('Error: $e');
}
}
FileMode.write opens the file for writing, creating it if needed. If the file exists, it's truncated to zero length first. We use openWrite for convenience.
$ dart main.dart File written successfully
Appending to a File
This example shows how to append content to an existing file.
import 'dart:io';
void main() async {
var file = File('log.txt');
try {
var sink = file.openWrite(mode: FileMode.append);
sink.write('${DateTime.now()}: New log entry\n');
await sink.close();
print('Log entry added');
} catch (e) {
print('Error: $e');
}
}
FileMode.append opens the file for writing at the end. If the file doesn't exist, it's created. Existing content is preserved and new content is added at the end.
$ dart main.dart Log entry added
Read-Write Access
This example demonstrates opening a file with both read and write access.
import 'dart:io';
void main() async {
var file = File('data.txt');
try {
var raf = await file.open(mode: FileMode.writeOnlyAppend);
await raf.writeString('Initial content\n');
await raf.close();
raf = await file.open(mode: FileMode.readWrite);
var content = await raf.readString();
await raf.writeString('Additional content\n');
await raf.close();
print('Updated content:\n$content');
} catch (e) {
print('Error: $e');
}
}
FileMode.readWrite allows both reading and writing to the file. The file pointer starts at the beginning, letting us read existing content before writing.
$ dart main.dart Updated content: Initial content
Exclusive File Creation
This example shows how to create a new file exclusively.
import 'dart:io';
void main() async {
var file = File('newfile.txt');
try {
var raf = await file.open(mode: FileMode.writeOnly);
await raf.writeString('This is a new file\n');
await raf.close();
print('File created exclusively');
} on FileSystemException catch (e) {
print('Error: ${e.message}');
}
}
FileMode.writeOnly creates a new file, failing if it already exists. This is useful when you need to ensure you're not overwriting an existing file.
$ dart main.dart File created exclusively
Best Practices
- Choose carefully: Select the mode that matches your needs
- Error handling: Always handle potential file system errors
- Resource cleanup: Close files when done to release resources
- Atomic operations: Use appropriate modes for data safety
Source
This tutorial covered Dart's FileMode class with practical examples showing different file access patterns and their appropriate use cases.
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