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rel="noopener">API reference</a></li><li class="nav-item"><a class="nav-link" href="https://medium.com/dartlang" target="_blank" rel="noopener">Blog</a></li><li class="nav-item"><a class="nav-link" href="https://dartpad.dev" target="_blank" rel="noopener">DartPad (online editor)</a></li><li class="nav-item"><a class="nav-link" href="https://flutter.dev" target="_blank" rel="noopener">Flutter</a></li><li class="nav-item"><a class="nav-link" href="https://pub.dev" target="_blank" rel="noopener">Package 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="#sound-and-unsound-null-safety">Sound and unsound null safety</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#migrating-incrementally">Migrating incrementally</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#using-the-migration-tool">Using the migration tool</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#migrating-by-hand">Migrating by hand</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#testing-or-running-mixed-version-programs">Testing or running mixed-version programs</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Unsound 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="#sound-and-unsound-null-safety">Sound and unsound null safety</a></li><li class="toc-entry"><a href="#migrating-incrementally">Migrating incrementally</a><ul><li class="toc-entry"><a href="#using-the-migration-tool">Using the migration tool</a></li><li class="toc-entry"><a href="#migrating-by-hand">Migrating by hand</a></li></ul></li><li class="toc-entry"><a href="#testing-or-running-mixed-version-programs">Testing or running mixed-version programs</a></li></ul></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">merge_type</i> <span>Version note</span></div><div class="alert-content"><p>Dart 3 and later does not support code without null safety or with unsound null safety. All code must be soundly null safe. To learn more, check out the <a href="https://github.com/dart-lang/sdk/issues/49530">Dart 3 sound null safety tracking issue</a>.</p></div></aside><p>A Dart program may contain some libraries that are <a href="/null-safety">null safe</a> and some that aren't. These <strong>mixed-version programs</strong> rely on <strong>unsound null safety</strong>.</p><p>The ability to mix <a href="/guides/language/evolution#language-versioning">language versions</a> frees package maintainers to migrate their code, with the knowledge that even legacy users can get new bug fixes and other improvements. However, mixed-version programs don't get all the advantages that null safety can bring.</p><p>This page describes the differences between sound and unsound null safety, with the goal of helping you decide when to migrate to null safety. After the conceptual discussion are instructions for migrating incrementally, followed by details on testing and running mixed-version programs.</p><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>We recommend that, if possible, you wait for dependencies to migrate before you migrate your package. For details, see the <a href="/null-safety/migration-guide">migration guide</a>.</p></div></aside><div class="header-wrapper"><h2 id="sound-and-unsound-null-safety">Sound and unsound null safety</h2><a class="heading-link" href="#sound-and-unsound-null-safety" aria-label="Link to 'Sound and unsound null safety' section">#</a></div><p>Dart provides sound null safety through a combination of static and runtime checks. Each Dart library that opts in to null safety gets all the <em>static</em> checks, with stricter compile-time errors. This is true even in a mixed-version program that contains null-unsafe libraries. You start getting these benefits as soon as you start migrating some of your code to null safety.</p><p>However, a mixed-version program can't have the <em>runtime</em> soundness guarantees that a fully null-safe app has. It's possible for <code>null</code> to leak out of the null-unsafe libraries into the null-safe code, because preventing that would break the existing behavior of the unmigrated code.</p><p>To maintain runtime compatibility with legacy libraries while offering soundness to completely null-safe programs, Dart tools support two modes:</p><ul><li><p>Mixed-version programs run with <strong>unsound null safety</strong>. It's possible for <code>null</code> reference errors to occur at runtime, but only because a <code>null</code> or nullable type escaped from some null-unsafe library and got into null-safe code.</p></li><li><p>When a program is fully migrated and <em>all</em> its libraries are null safe, then it runs with <strong>sound null safety</strong>, with all of the guarantees and compiler optimizations that soundness enables.</p></li></ul><p>Sound null safety is what you want if possible. Dart tools automatically run your program in sound mode if the main entrypoint library of your program has opted into null safety. If you import a null-unsafe library, the tools print a warning to let you know that they can only run with unsound null safety.</p><div class="header-wrapper"><h2 id="migrating-incrementally">Migrating incrementally</h2><a class="heading-link" href="#migrating-incrementally" aria-label="Link to 'Migrating incrementally' section">#</a></div><p>Because Dart supports mixed-version programs, you can migrate one library (generally one Dart file) at a time, while still being able to run your program and its tests.</p><p>We recommend that you <strong>first migrate leaf libraries</strong>鈥攍ibraries that don't import other files from the package. Then migrate libraries that directly depend on the leaf libraries. End by migrating the libraries that have the most intra-package dependencies.</p><p>For example, say you have a <code>lib/src/util.dart</code> file that imports other (null-safe) packages and core libraries, but that doesn't have any <code>import '<local_path>'</code> directives. Consider migrating <code>util.dart</code> first, and then migrating files that depend only on <code>util.dart</code>. If any libraries have cyclic imports (for example, A imports B which imports C, and C imports A), consider migrating those libraries together.</p><div class="header-wrapper"><h3 id="using-the-migration-tool">Using the migration tool</h3><a class="heading-link" href="#using-the-migration-tool" aria-label="Link to 'Using the migration tool' section">#</a></div><p>You can migrate incrementally using the <a href="/null-safety/migration-guide#step2-migrate">migration tool</a>. To opt out files or directories, click the green checkbox. In the following screenshot, all files in the <code>bin</code> directory are opted out.</p><p><img src="/assets/img/null-safety/migration-tool-incremental.png" alt="Screenshot of file viewer in migration tool"></p><p>Each opted out file will be unchanged except for a 2.9 <a href="/guides/language/evolution#per-library-language-version-selection">language version comment</a>. You can later run <code>dart migrate</code> again to continue the migration. Any files that are already migrated feature a disabled checkbox: you cannot un-migrate a file once it has been migrated.</p><div class="header-wrapper"><h3 id="migrating-by-hand">Migrating by hand</h3><a class="heading-link" href="#migrating-by-hand" aria-label="Link to 'Migrating by hand' section">#</a></div><p>If you want to incrementally migrate a package by hand, follow these steps:</p><ol><li><p>Edit the package's <code>pubspec.yaml</code> file, setting the minimum SDK constraint to at least <code>2.12.0</code>:</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></li><li><p>Regenerate the <a href="https://github.com/dart-lang/language/blob/main/accepted/2.8/language-versioning/package-config-file-v2.md">package configuration file</a>:</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></code></pre></div></div><p>Running <code>dart pub get</code> with a lower SDK constraint of <code>2.12.0</code> sets the default language version of every library in the package to 2.12, opting them all in to null safety.</p></li><li><p>Open the package in your IDE.<br>You're likely to see a lot of analysis errors. That's OK.</p></li><li><p>Add a <a href="/guides/language/evolution#per-library-language-version-selection">language version comment</a> to the top of any Dart files that you don't want to consider during your current migration:</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">// @dart=2.9</span></span></code></pre></div></div><p>Using language version 2.9 for a library that's in a 2.12 package can reduce analysis errors (red squiggles) coming from unmigrated code. However, <strong>unsound null safety reduces the information the analyzer can use.</strong> For example, the analyzer might assume a parameter type is non-nullable, even though a 2.9 file might pass in a null value.</p></li><li><p>Migrate the code of each Dart file, using the analyzer to identify static errors.<br>Eliminate static errors by adding <code>?</code>, <code>!</code>, <code>required</code>, and <code>late</code>, as needed.</p></li></ol><div class="header-wrapper"><h2 id="testing-or-running-mixed-version-programs">Testing or running mixed-version programs</h2><a class="heading-link" href="#testing-or-running-mixed-version-programs" aria-label="Link to 'Testing or running mixed-version programs' section">#</a></div><p>To test or run mixed-version code, you need to disable sound null safety. You can do this in two ways:</p><ul><li><p>Disable sound null safety using the <code>--no-sound-null-safety</code> flag to the <code>dart</code> or <code>flutter</code> command:</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 --no-sound-null-safety run</span></span> <span class="line"><span style="color:#222222">$ flutter run --no-sound-null-safety</span></span></code></pre></div></div></li><li><p>Alternatively, set the language version in the entrypoint鈥攖he file that contains <code>main()</code> function鈥攖o 2.9. In Flutter apps, this file is often named <code>lib/main.dart</code>. In command-line apps, this file is often named <code>bin/<packageName>.dart</code>. You can also opt out files under <code>test</code>, because they are also entrypoints. Example:</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">// @dart=2.9</span></span> <span class="line"><span style="color:#D43324">import</span><span style="color:#11796D"> 'src/my_app.dart'</span><span style="color:#222222">;</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> main</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#6E6E70"> //...</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div></li></ul><p>Opting out tests using either of these mechanisms can be useful for testing <strong>during</strong> your incremental migration process, but doing so means that you aren't testing your code with full null safety enabled. It's important to opt your tests back <em>in</em> to null safety when you've finished the incremental migration of your libraries.</p><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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