exists

Learn to check file existence with File.exists.

Checking file existence

File.existsSync() returns a bool telling whether a file exists. Checking before reading prevents a missing file error.

Example: if (f.existsSync()) { print(f.readAsStringSync()); }. If the file exists it reads the content; otherwise it skips.

The async exists() returns a Future<bool>. For small programs the synchronous version is simpler.

Handling a missing file

If the file is missing readAsStringSync() throws. To prevent this, check with existsSync() first and use a default value when missing.

Example: var content = f.existsSync() ? f.readAsStringSync() : 'empty';. If the file is missing it returns 'empty'; the program does not crash.

This pattern is common for config files: when the file is missing a default setting is used.

Exercise Print existence

Import the library with import 'dart:io';. In main create File('test.txt'); write with writeAsStringSync('data'). Print the result of existsSync() with print.

Show example solution

import 'dart:io';

void main() {
  var f = File('test.txt');
  f.writeAsStringSync('data');
  print(f.existsSync());
}

Test yourself

What does existsSync() return?
- A string
- A bool
- void
- A Future
Show answer

A bool — existsSync() returns a bool.

The existsSync call

var f = File('not.txt');
if (f.____()) {
  print('var');
}

Show answer

var f = File('not.txt');
if (f.existsSync()) {
  print('var');
}

Existence check parts

existsSync()

Sync check
exists()

Async check
Future<bool>

Async return
if (f.existsSync())

Condition

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