fix: add informative messages to error classes

CHANGELOG.md

@@ -14,6 +14,11 @@ Release notes are available on [github][notes].
 - Error messages now include the variable name for easier debugging
 - [fix] `load(isOptional: true)` no longer discards successfully loaded base file when an override file is missing or empty (fixes #70, #93, #101, #125)
 - [fix] `clean()` now resets `isInitialized` to `false`, so accessing `env` after `clean()` correctly throws `NotInitializedError`
+- [fix] Error classes (`NotInitializedError`, `FileNotFoundError`, `EmptyEnvFileError`) now include informative messages in `toString()` instead of the unhelpful `Instance of 'ClassName'` (fixes #72, #127; improves diagnostics for #59, #89)
+- `FileNotFoundError` and `EmptyEnvFileError` now carry the filename when available
+
+### Note on error message improvements
+`NotInitializedError`, `FileNotFoundError`, and `EmptyEnvFileError` now override `toString()` with actionable messages (e.g., `FileNotFoundError: Environment file ".env" not found. Ensure the file exists and is listed under assets in pubspec.yaml.`). This is **not a breaking change** — the class names and hierarchy are unchanged, so existing `on FileNotFoundError` catch clauses continue to work. `FileNotFoundError` now accepts an optional positional `filename` parameter, and `EmptyEnvFileError` accepts an optional named `filename` parameter; both default to `null` for backward compatibility.
 
 ### Note on release-build behavior change
 In **debug mode**, behavior is unchanged — `AssertionError` was thrown before, `AssertionError` is thrown now.

lib/src/dotenv.dart

@@ -216,11 +216,11 @@ class DotEnv {
       WidgetsFlutterBinding.ensureInitialized();
       var envString = await rootBundle.loadString(filename);
       if (envString.isEmpty) {
-        throw EmptyEnvFileError();
+        throw EmptyEnvFileError(filename: filename);
       }
       return envString.split('\n');
     } on FlutterError {
-      throw FileNotFoundError();
+      throw FileNotFoundError(filename);
     }
   }
 

lib/src/errors.dart

@@ -1,5 +1,28 @@
-class NotInitializedError extends Error {}
+class NotInitializedError extends Error {
+  @override
+  String toString() =>
+      'NotInitializedError: DotEnv has not been initialized. '
+      'Call load() or loadFromString() before accessing env variables.';
+}
 
-class FileNotFoundError extends Error {}
+class FileNotFoundError extends Error {
+  final String? filename;
+  FileNotFoundError([this.filename]);
 
-class EmptyEnvFileError extends Error {}
+  @override
+  String toString() => filename != null
+      ? 'FileNotFoundError: Environment file "$filename" not found. '
+          'Ensure the file exists and is listed under assets in pubspec.yaml.'
+      : 'FileNotFoundError: Environment file not found. '
+          'Ensure the file exists and is listed under assets in pubspec.yaml.';
+}
+
+class EmptyEnvFileError extends Error {
+  final String? filename;
+  EmptyEnvFileError({this.filename});
+
+  @override
+  String toString() => filename != null
+      ? 'EmptyEnvFileError: Environment file "$filename" is empty.'
+      : 'EmptyEnvFileError: The provided env string is empty.';
+}

test/errors_test.dart

@@ -0,0 +1,82 @@
+import 'package:flutter_dotenv/flutter_dotenv.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('NotInitializedError', () {
+    test('toString contains actionable guidance', () {
+      final error = NotInitializedError();
+      expect(error.toString(), contains('NotInitializedError'));
+      expect(error.toString(), contains('load()'));
+      expect(error.toString(), contains('loadFromString()'));
+    });
+
+    test('is thrown when accessing env before initialization', () {
+      final env = DotEnv();
+      expect(() => env.env, throwsA(isA<NotInitializedError>()));
+    });
+
+    test('thrown error message does not reference global singleton', () {
+      final env = DotEnv();
+      try {
+        env.env;
+        fail('should have thrown');
+      } on NotInitializedError catch (e) {
+        expect(e.toString(), contains('load()'));
+        expect(e.toString(), isNot(contains('dotenv.load')));
+      }
+    });
+  });
+
+  group('FileNotFoundError', () {
+    test('toString contains the filename when provided', () {
+      final error = FileNotFoundError('.env');
+      expect(error.toString(), contains('FileNotFoundError'));
+      expect(error.toString(), contains('.env'));
+      expect(error.toString(), contains('pubspec.yaml'));
+    });
+
+    test('toString provides generic message when no filename given', () {
+      final error = FileNotFoundError();
+      expect(error.toString(), contains('FileNotFoundError'));
+      expect(error.toString(), contains('not found'));
+    });
+
+    test('filename field is accessible', () {
+      expect(FileNotFoundError('.env.production').filename, '.env.production');
+      expect(FileNotFoundError().filename, isNull);
+    });
+  });
+
+  group('EmptyEnvFileError from loadFromString', () {
+    test('thrown error message describes empty string context', () {
+      final env = DotEnv();
+      try {
+        env.loadFromString(envString: '');
+        fail('should have thrown');
+      } on EmptyEnvFileError catch (e) {
+        expect(e.toString(), contains('empty'));
+        expect(e.filename, isNull);
+      }
+    });
+  });
+
+  group('EmptyEnvFileError', () {
+    test('toString contains the filename when provided', () {
+      final error = EmptyEnvFileError(filename: '.env');
+      expect(error.toString(), contains('EmptyEnvFileError'));
+      expect(error.toString(), contains('.env'));
+      expect(error.toString(), contains('empty'));
+    });
+
+    test('toString provides generic message when no filename given', () {
+      final error = EmptyEnvFileError();
+      expect(error.toString(), contains('EmptyEnvFileError'));
+      expect(error.toString(), contains('empty'));
+    });
+
+    test('filename field is accessible', () {
+      expect(EmptyEnvFileError(filename: '.env').filename, '.env');
+      expect(EmptyEnvFileError().filename, isNull);
+    });
+  });
+}