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site</a></li></ul></li></ul></div></div><main id="page-content"><div id="site-toc--side" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#introduction-through-examples">Introduction through examples</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#null-safety-principles">Null safety principles</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#dart-3-and-null-safety">Dart 3 and null safety</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#enable-null-safety">Dart 2.x and null safety</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#migrate">Migrating existing code</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#where-to-learn-more">Where to learn more</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Sound null safety</h1></div><div id="site-toc--inline" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry"><a href="#introduction-through-examples">Introduction through examples</a></li><li class="toc-entry"><a href="#null-safety-principles">Null safety principles</a></li><li class="toc-entry"><a href="#dart-3-and-null-safety">Dart 3 and null safety</a></li><li class="toc-entry"><a href="#enable-null-safety">Dart 2.x and null safety</a></li><li class="toc-entry"><a href="#migrate">Migrating existing code</a></li><li class="toc-entry"><a href="#where-to-learn-more">Where to learn more</a></li></ul></div><p>The Dart language enforces sound null safety.</p><p>Null safety prevents errors that result from unintentional access of variables set to <code>null</code>.</p><p>For example, if a method expects an integer but receives <code>null</code>, your app causes a runtime error. This type of error, a null dereference error, can be difficult to debug.</p><p>With sound null safety, all variables require a value. This means Dart considers all variables <em>non-nullable</em>. You can assign values of the declared type only, like <code>int i=42</code>. You can never assign a value of <code>null</code> to default variable types. To specify that a variable type can have a <code>null</code> value, add a <code>?</code> after the type annotation: <code>int? i</code>. These specific types can contain either a <code>null</code> <em>or</em> a value of the defined type.</p><p>Sound null safety changes potential <strong>runtime errors</strong> into <strong>edit-time</strong> analysis errors. With null safety, the Dart analyzer and compilers flag if a non-nullable variable has either:</p><ul><li>Not been initialized with a non-null value</li><li>Been assigned a <code>null</code> value.</li></ul><p>These checks allow you to fix these errors <em>before</em> deploying your app.</p><div class="header-wrapper"><h2 id="introduction-through-examples">Introduction through examples</h2><a class="heading-link" href="#introduction-through-examples" aria-label="Link to 'Introduction through examples' section">#</a></div><p>With null safety, none of the variables in the following code can be <code>null</code>:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#6E6E70">// With null safety, none of these can ever be null.</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> i = </span><span style="color:#11796D">42</span><span style="color:#222222">; </span><span style="color:#6E6E70">// Inferred to be an int.</span></span> <span class="line"><span style="color:#0468D7">String</span><span style="color:#222222"> name = </span><span style="color:#6200EE">getFileName</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324">final</span><span style="color:#222222"> b = </span><span style="color:#0468D7">Foo</span><span style="color:#222222">();</span></span></code></pre></div></div><p><a id="creating-variables"></a> To indicate that a variable might have the value <code>null</code>, just add <code>?</code> to its type declaration:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#0468D7">int</span><span style="color:#222222">? aNullableInt = </span><span style="color:#11796D">null</span><span style="color:#222222">;</span></span></code></pre></div></div><ul><li>To try some interactive examples, try out some of the null-safety orientated examples in the <a href="/resources/dart-cheatsheet">Dart cheatsheet</a>.</li><li>To learn more about null safety, check out <a href="/null-safety/understanding-null-safety">Understanding null safety</a>.</li></ul><div class="header-wrapper"><h2 id="null-safety-principles">Null safety principles</h2><a class="heading-link" href="#null-safety-principles" aria-label="Link to 'Null safety principles' section">#</a></div><p>Dart supports null safety using the following two core design principles:</p><ul><li><p><strong>Non-nullable by default</strong>. Unless you explicitly tell Dart that a variable can be null, it's considered non-nullable. This default was chosen after research found that non-null was by far the most common choice in APIs.</p></li><li><p><strong>Fully sound</strong>. Dart's null safety is sound, which enables compiler optimizations. If the type system determines that something isn't null, then that thing can <em>never</em> be null. Once you migrate your whole project and its dependencies to null safety, you reap the full benefits of soundness—not only fewer bugs, but smaller binaries and faster execution.</p></li></ul><div class="header-wrapper"><h2 id="dart-3-and-null-safety">Dart 3 and null safety</h2><a class="heading-link" href="#dart-3-and-null-safety" aria-label="Link to 'Dart 3 and null safety' section">#</a></div><p>Dart 3 has built-in sound null safety. Dart 3 prevents code without it from running.</p><p>To learn how to migrate to Dart 3, check out the <a href="/resources/dart-3-migration">Dart 3 migration guide</a>. Packages developed without null safety support cause issues when resolving dependencies:</p><div class="code-block-wrapper language-console"><div class="code-block-body"><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#222222">$ dart pub get</span></span> <span class="line"></span> <span class="line"><span style="color:#222222">Because pkg1 doesn't support null safety, version solving failed.</span></span> <span class="line"><span style="color:#222222">The lower bound of "sdk: '>=2.9.0 <3.0.0'" must be 2.12.0 or higher to enable null safety.</span></span></code></pre></div></div><p>Libraries incompatible with Dart 3 cause analysis or compilation errors.</p><div class="code-block-wrapper language-console"><div class="code-block-body"><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#222222">$ dart analyze </span><span style="color:#6200EE">.</span></span> <span class="line"><span style="color:#222222">Analyzing .... 0.6s</span></span> <span class="line"></span> <span class="line"><span style="color:#222222"> error • lib/pkg1.dart:1:1 • The language version must be >=2.12.0. </span></span> <span class="line"><span style="color:#222222"> Try removing the language version override and migrating the code.</span></span> <span class="line"><span style="color:#222222"> • illegal_language_version_override</span></span></code></pre></div></div><div class="code-block-wrapper language-console"><div class="code-block-body"><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#222222">$ dart run bin/my_app.dart</span></span> <span class="line"><span style="color:#222222">../pkg1/lib/pkg1.dart:1:1: Error: Library doesn't support null safety.</span></span> <span class="line"><span style="color:#222222">// @dart=2.9</span></span> <span class="line"><span style="color:#222222">^^^^^^^^^^^^</span></span></code></pre></div></div><p>To resolve these issues:</p><ol><li>Check for <a href="/null-safety/migration-guide#check-dependency-status">null safe versions</a> of any packages you installed from pub.dev</li><li><a href="#migrate">migrate</a> all of your source code to use sound null safety.</li></ol><p>Dart 3 can be found in the stable channels for Dart and Flutter. To learn more, check out <a href="/get-dart/archive">the download page</a> for details. To test your code for Dart 3 compatibility, use Dart 3 or later.</p><div class="code-block-wrapper language-console"><div class="code-block-body"><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#222222">$ dart --version </span><span style="color:#6E6E70"># make sure this reports 3.0.0-417.1.beta or higher</span></span> <span class="line"><span style="color:#222222">$ dart pub get / flutter pub get </span><span style="color:#6E6E70"># this should resolve without issues</span></span> <span class="line"><span style="color:#222222">$ dart analyze / flutter analyze </span><span style="color:#6E6E70"># this should pass without errors</span></span></code></pre></div></div><p>If the <code>pub get</code> step fails, check the <a href="/null-safety/migration-guide#check-dependency-status">status of the dependencies</a>.</p><p>If the <code>analyze</code> step fails, update your code to resolve the issues listed by the analyzer.</p><div class="header-wrapper"><h2 id="enable-null-safety">Dart 2.x and null safety</h2><a class="heading-link" href="#enable-null-safety" aria-label="Link to 'Dart 2.x and null safety' section">#</a></div><p>From Dart 2.12 to 2.19, you need to enable null safety. You cannot use null safety in SDK versions earlier than Dart 2.12.</p><p><a id="constraints"></a> To enable sound null safety, set the <a href="/tools/pub/pubspec#sdk-constraints">SDK constraint lower-bound</a> to a <a href="/guides/language/evolution#language-versioning">language version</a> of 2.12 or later. For example, your <code>pubspec.yaml</code> file might have the following constraints:</p><div class="code-block-wrapper language-yaml"><div class="code-block-body"><span class="code-block-language" title="Language yaml">yaml</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#0468D7">environment</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#0468D7"> sdk</span><span style="color:#222222">: </span><span style="color:#11796D">'>=2.12.0 <3.0.0'</span></span></code></pre></div></div><div class="header-wrapper"><h2 id="migrate">Migrating existing code</h2><a class="heading-link" href="#migrate" aria-label="Link to 'Migrating existing code' section">#</a></div><aside class="alert alert-warning"><div class="alert-header"><i class="material-symbols" aria-hidden="true">warning</i> <span>Warning</span></div><div class="alert-content"><p>Dart 3 removes the <code>dart migrate</code> tool. If you need help migrating your code, run the tool with the 2.19 SDK, then upgrade to Dart 3.</p><p>You can migrate without the tool, but it involves hand editing code.</p></div></aside><p>Dart code written without null safety support can be migrated to use null safety. We recommend using the <code>dart migrate</code> tool, included in the Dart SDK versions 2.12 to 2.19.</p><div class="code-block-wrapper language-console"><div class="code-block-body"><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#222222">$ cd my_app</span></span> <span class="line"><span style="color:#222222">$ dart migrate</span></span></code></pre></div></div><p>To learn how to migrate your code to null safety, check out the <a href="/null-safety/migration-guide">migration guide</a>.</p><div class="header-wrapper"><h2 id="where-to-learn-more">Where to learn more</h2><a class="heading-link" href="#where-to-learn-more" aria-label="Link to 'Where to learn more' section">#</a></div><p>To learn more about null safety, check out the following resources:</p><ul><li><a href="/null-safety/understanding-null-safety">Understanding null safety</a></li><li><a href="/null-safety/migration-guide">Migration guide for existing code</a></li><li><a href="/null-safety/faq">Null safety FAQ</a></li><li><a href="https://github.com/dart-lang/samples/tree/main/null_safety/calculate_lix">Null safety sample code</a></li></ul><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. Page last updated on 2024-06-10.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/null-safety/index.md" target="_blank" rel="noopener">View source</a> <span>or </span><a href="https://github.com/dart-lang/site-www/issues/new?template=1_page_issue.yml&page-url=https://dart.dev/null-safety/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/null-safety/index.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main><footer id="page-footer"><div class="footer-section footer-main"><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart" width="164"></a><div class="footer-social-links"><a href="https://medium.com/dartlang" target="_blank" rel="noopener" title="Medium blog"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="GitHub"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="X (Twitter)"><svg><use href="/assets/img/social/x.svg#x"></use></svg></a></div></div><div class="footer-section footer-tray"><div class="footer-licenses">Except as otherwise noted, this site is licensed under a <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>, and code samples are licensed under the <a href="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</a>.</div><div class="footer-utility-links"><ul><li><a href="/terms" title="Terms of use">Terms</a></li><li><a href="https://policies.google.com/privacy" target="_blank" rel="noopener" title="Privacy policy">Privacy</a></li><li><a href="/security" title="Security philosophy and practices">Security</a></li></ul></div></div></footer></body></html>