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href="/libraries/async/futures-error-handling"><div><span>Futures and error handling</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/using-streams"><div><span>Using streams</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/creating-streams"><div><span>Creating streams</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-3" role="button" aria-expanded="false" aria-controls="-sidenav-3"><span>Effective Dart</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-3"><li class="nav-item"><a class="nav-link" href="/effective-dart"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/style"><div><span>Style</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/documentation"><div><span>Documentation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/usage"><div><span>Usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/design"><div><span>Design</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-4" role="button" aria-expanded="false" aria-controls="-sidenav-4"><span>Packages</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-4"><li class="nav-item"><a class="nav-link" href="/tools/pub/packages"><div><span>How to use packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/useful-packages"><div><span>Commonly used packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/create-packages"><div><span>Creating packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/publishing"><div><span>Publishing packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/writing-package-pages"><div><span>Writing package pages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/workspaces"><div><span>Workspaces (monorepo support)</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-4-7" role="button" aria-expanded="false" aria-controls="-sidenav-4-7"><span>Package reference</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-4-7"><li class="nav-item"><a class="nav-link" href="/tools/pub/dependencies"><div><span>Dependencies</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/glossary"><div><span>Glossary</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/package-layout"><div><span>Package layout conventions</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/environment-variables"><div><span>Pub environment variables</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/pubspec"><div><span>Pubspec file</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/troubleshoot"><div><span>Troubleshooting pub</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/verified-publishers"><div><span>Verified publishers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/security-advisories"><div><span>Security advisories</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/versioning"><div><span>Versioning</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/custom-package-repositories"><div><span>Custom package repositories</span></div></a></li></ul></li><li class="nav-item"><a class="nav-link" href="/tools/pub/private-files"><div><span>What not to commit</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-5" role="button" aria-expanded="false" aria-controls="-sidenav-5"><span>Development</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5"><li class="nav-item"><a class="nav-link" href="/libraries/serialization/json"><div><span>JSON serialization</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/language/number-representation"><div><span>Number representation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/google-apis"><div><span>Google APIs</span></div></a></li><li class="nav-item"><a class="nav-link" href="/multiplatform-apps"><div><span>Multi-platform apps</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-5-5" role="button" aria-expanded="false" aria-controls="-sidenav-5-5"><span>Command-line & server apps</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5-5"><li class="nav-item"><a class="nav-link" href="/server"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/get-started"><div><span>Get started</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/cmdline"><div><span>Write command-line apps</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/fetch-data"><div><span>Fetch data from the internet</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/httpserver"><div><span>Write HTTP servers</span></div></a></li><li 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class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#if">If</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#if-case">If-case</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#switch-statements">Switch statements</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#switch-expressions">Switch expressions</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#exhaustiveness-checking">Exhaustiveness checking</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#guard-clause">Guard clause</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Branches</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="#if">If</a><ul><li class="toc-entry"><a href="#if-case">If-case</a></li></ul></li><li class="toc-entry"><a href="#switch-statements">Switch statements</a><ul><li class="toc-entry"><a href="#switch-expressions">Switch expressions</a></li><li class="toc-entry"><a href="#exhaustiveness-checking">Exhaustiveness checking</a></li></ul></li><li class="toc-entry"><a href="#guard-clause">Guard clause</a></li></ul></div><p>This page shows how you can control the flow of your Dart code using branches:</p><ul><li><code>if</code> statements and elements</li><li><code>if-case</code> statements and elements</li><li><code>switch</code> statements and expressions</li></ul><p>You can also manipulate control flow in Dart using:</p><ul><li><a href="/language/loops">Loops</a>, like <code>for</code> and <code>while</code></li><li><a href="/language/error-handling">Exceptions</a>, like <code>try</code>, <code>catch</code>, and <code>throw</code></li></ul><div class="header-wrapper"><h2 id="if">If</h2><a class="heading-link" href="#if" aria-label="Link to 'If' section">#</a></div><p>Dart supports <code>if</code> statements with optional <code>else</code> clauses. The condition in parentheses after <code>if</code> must be an expression that evaluates to a <a href="/language/built-in-types#booleans">boolean</a>:</p> <?code-excerpt "language/lib/control_flow/branches.dart (if-else)"?> <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:#D43324">if</span><span style="color:#222222"> (</span><span style="color:#6200EE">isRaining</span><span style="color:#222222">()) {</span></span> <span class="line"><span style="color:#222222"> you.</span><span style="color:#6200EE">bringRainCoat</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">else</span><span style="color:#D43324"> if</span><span style="color:#222222"> (</span><span style="color:#6200EE">isSnowing</span><span style="color:#222222">()) {</span></span> <span class="line"><span style="color:#222222"> you.</span><span style="color:#6200EE">wearJacket</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">else</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#222222"> car.</span><span style="color:#6200EE">putTopDown</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>To learn how to use <code>if</code> in an expression context, check out <a href="/language/operators#conditional-expressions">Conditional expressions</a>.</p><div class="header-wrapper"><h3 id="if-case">If-case</h3><a class="heading-link" href="#if-case" aria-label="Link to 'If-case' section">#</a></div><p>Dart <code>if</code> statements support <code>case</code> clauses followed by a <a href="/language/patterns">pattern</a>:</p> <?code-excerpt "language/lib/control_flow/branches.dart (if-case)"?> <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:#D43324">if</span><span style="color:#222222"> (pair </span><span style="color:#D43324">case</span><span style="color:#222222"> [</span><span style="color:#0468D7">int</span><span style="color:#222222"> x, </span><span style="color:#0468D7">int</span><span style="color:#222222"> y]) </span><span style="color:#D43324">return</span><span style="color:#0468D7"> Point</span><span style="color:#222222">(x, y);</span></span></code></pre></div></div><p>If the pattern matches the value, then the branch executes with any variables the pattern defines in scope.</p><p>In the previous example, the list pattern <code>[int x, int y]</code> matches the value <code>pair</code>, so the branch <code>return Point(x, y)</code> executes with the variables that the pattern defined, <code>x</code> and <code>y</code>.</p><p>Otherwise, control flow progresses to the <code>else</code> branch to execute, if there is one:</p> <?code-excerpt "language/lib/control_flow/branches.dart (if-case-else)"?> <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:#D43324">if</span><span style="color:#222222"> (pair </span><span style="color:#D43324">case</span><span style="color:#222222"> [</span><span style="color:#0468D7">int</span><span style="color:#222222"> x, </span><span style="color:#0468D7">int</span><span style="color:#222222"> y]) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Was coordinate array </span><span style="color:#11796D">$</span><span style="color:#222222">x</span><span style="color:#11796D">,</span><span style="color:#11796D">$</span><span style="color:#222222">y</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">else</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> throw</span><span style="color:#0468D7"> FormatException</span><span style="color:#222222">(</span><span style="color:#11796D">'Invalid coordinates.'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The if-case statement provides a way to match and <a href="/language/patterns#destructuring">destructure</a> against a <em>single</em> pattern. To test a value against <em>multiple</em> patterns, use <a href="#switch">switch</a>.</p><aside class="alert alert-info"><div class="alert-header"><span class="material-symbols" aria-hidden="true">merge_type</span> <span>Version note</span></div><div class="alert-content"><p>Case clauses in if statements require a <a href="/resources/language/evolution#language-versioning">language version</a> of at least 3.0.</p></div></aside><p><a id="switch"></a></p><div class="header-wrapper"><h2 id="switch-statements">Switch statements</h2><a class="heading-link" href="#switch-statements" aria-label="Link to 'Switch statements' section">#</a></div><p>A <code>switch</code> statement evaluates a value expression against a series of cases. Each <code>case</code> clause is a <a href="/language/patterns">pattern</a> for the value to match against. You can use <a href="/language/pattern-types">any kind of pattern</a> for a case.</p><p>When the value matches a case's pattern, the case body executes. Non-empty <code>case</code> clauses jump to the end of the switch after completion. They do not require a <code>break</code> statement. Other valid ways to end a non-empty <code>case</code> clause are a <a href="/language/loops#break-and-continue"><code>continue</code></a>, <a href="/language/error-handling#throw"><code>throw</code></a>, or <a href="/language/functions#return-values"><code>return</code></a> statement.</p><p>Use a <code>default</code> or <a href="/language/pattern-types#wildcard">wildcard <code>_</code></a> clause to execute code when no <code>case</code> clause matches:</p> <?code-excerpt "language/lib/control_flow/branches.dart (switch)"?> <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:#D43324">var</span><span style="color:#222222"> command = </span><span style="color:#11796D">'OPEN'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#D43324">switch</span><span style="color:#222222"> (command) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'CLOSED'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeClosed</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'PENDING'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executePending</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'APPROVED'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeApproved</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'DENIED'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeDenied</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'OPEN'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeOpen</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> default</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeUnknown</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p><a id="switch-share"></a></p><p>Empty cases fall through to the next case, allowing cases to share a body. For an empty case that does not fall through, use <a href="/language/loops#break-and-continue"><code>break</code></a> for its body. For non-sequential fall-through, you can use a <a href="/language/loops#break-and-continue"><code>continue</code> statement</a> and a label:</p> <?code-excerpt "language/lib/control_flow/branches.dart (switch-empty)"?> <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:#D43324">switch</span><span style="color:#222222"> (command) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'OPEN'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeOpen</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> continue</span><span style="color:#222222"> newCase; </span><span style="color:#6E6E70">// Continues executing at the newCase label.</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'DENIED'</span><span style="color:#222222">: </span><span style="color:#6E6E70">// Empty case falls through.</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'CLOSED'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeClosed</span><span style="color:#222222">(); </span><span style="color:#6E6E70">// Runs for both DENIED and CLOSED,</span></span> <span class="line"></span> <span class="line"><span style="color:#222222"> newCase:</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> 'PENDING'</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> executeNowClosed</span><span style="color:#222222">(); </span><span style="color:#6E6E70">// Runs for both OPEN and PENDING.</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>You can use <a href="/language/patterns#or-pattern-switch">logical-or patterns</a> to allow cases to share a body or a guard. To learn more about patterns and case clauses, check out the patterns documentation on <a href="/language/patterns#switch-statements-and-expressions">Switch statements and expressions</a>.</p><div class="header-wrapper"><h3 id="switch-expressions">Switch expressions</h3><a class="heading-link" href="#switch-expressions" aria-label="Link to 'Switch expressions' section">#</a></div><p>A <code>switch</code> <em>expression</em> produces a value based on the expression body of whichever case matches. You can use a switch expression wherever Dart allows expressions, <em>except</em> at the start of an expression statement. For 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:#D43324">var</span><span style="color:#222222"> x = </span><span style="color:#D43324">switch</span><span style="color:#222222"> (y) { ... };</span></span> <span class="line"></span> <span class="line"><span style="color:#6200EE">print</span><span style="color:#222222">(</span><span style="color:#D43324">switch</span><span style="color:#222222"> (x) { ... });</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">return</span><span style="color:#D43324"> switch</span><span style="color:#222222"> (x) { ... };</span></span></code></pre></div></div><p>If you want to use a switch at the start of an expression statement, use a <a href="#switch-statements">switch statement</a>.</p><p>Switch expressions allow you to rewrite a switch <em>statement</em> like this:</p> <?code-excerpt "language/lib/control_flow/branches.dart (switch-stmt)"?> <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">// Where slash, star, comma, semicolon, etc., are constant variables...</span></span> <span class="line"><span style="color:#D43324">switch</span><span style="color:#222222"> (charCode) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#222222"> slash || star || plus || minus: </span><span style="color:#6E6E70">// Logical-or pattern</span></span> <span class="line"><span style="color:#222222"> token = </span><span style="color:#D43324">operator</span><span style="color:#222222">(charCode);</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#222222"> comma || semicolon: </span><span style="color:#6E6E70">// Logical-or pattern</span></span> <span class="line"><span style="color:#222222"> token = </span><span style="color:#6200EE">punctuation</span><span style="color:#222222">(charCode);</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#222222"> >= digit0 && <= digit9: </span><span style="color:#6E6E70">// Relational and logical-and patterns</span></span> <span class="line"><span style="color:#222222"> token = </span><span style="color:#6200EE">number</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324"> default</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#D43324"> throw</span><span style="color:#0468D7"> FormatException</span><span style="color:#222222">(</span><span style="color:#11796D">'Invalid'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>Into an <em>expression</em>, like this:</p> <?code-excerpt "language/lib/control_flow/branches.dart (switch-exp)"?> <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:#222222">token = </span><span style="color:#D43324">switch</span><span style="color:#222222"> (charCode) {</span></span> <span class="line"><span style="color:#222222"> slash || star || plus || minus => </span><span style="color:#D43324">operator</span><span style="color:#222222">(charCode),</span></span> <span class="line"><span style="color:#222222"> comma || semicolon => </span><span style="color:#6200EE">punctuation</span><span style="color:#222222">(charCode),</span></span> <span class="line"><span style="color:#222222"> >= digit0 && <= digit9 => </span><span style="color:#6200EE">number</span><span style="color:#222222">(),</span></span> <span class="line"><span style="color:#222222"> _ => </span><span style="color:#D43324">throw</span><span style="color:#0468D7"> FormatException</span><span style="color:#222222">(</span><span style="color:#11796D">'Invalid'</span><span style="color:#222222">),</span></span> <span class="line"><span style="color:#222222">};</span></span></code></pre></div></div><p>The syntax of a <code>switch</code> expression differs from <code>switch</code> statement syntax:</p><ul><li>Cases <em>do not</em> start with the <code>case</code> keyword.</li><li>A case body is a single expression instead of a series of statements.</li><li>Each case must have a body; there is no implicit fallthrough for empty cases.</li><li>Case patterns are separated from their bodies using <code>=></code> instead of <code>:</code>.</li><li>Cases are separated by <code>,</code> (and an optional trailing <code>,</code> is allowed).</li><li>Default cases can <em>only</em> use <code>_</code>, instead of allowing both <code>default</code> and <code>_</code>.</li></ul><aside class="alert alert-info"><div class="alert-header"><span class="material-symbols" aria-hidden="true">merge_type</span> <span>Version note</span></div><div class="alert-content"><p>Switch expressions require a <a href="/resources/language/evolution#language-versioning">language version</a> of at least 3.0.</p></div></aside><div class="header-wrapper"><h3 id="exhaustiveness-checking">Exhaustiveness checking</h3><a class="heading-link" href="#exhaustiveness-checking" aria-label="Link to 'Exhaustiveness checking' section">#</a></div><p>Exhaustiveness checking is a feature that reports a compile-time error if it's possible for a value to enter a switch but not match any of the cases.</p> <?code-excerpt "language/lib/control_flow/branches.dart (exh-bool)"?> <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">// Non-exhaustive switch on bool?, missing case to match null possibility:</span></span> <span class="line"><span style="color:#D43324">switch</span><span style="color:#222222"> (nullableBool) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> true</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'yes'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#11796D"> false</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'no'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>A default case (<code>default</code> or <code>_</code>) covers all possible values that can flow through a switch. This makes a switch on any type exhaustive.</p><p><a href="/language/enums">Enums</a> and <a href="/language/class-modifiers#sealed">sealed types</a> are particularly useful for switches because, even without a default case, their possible values are known and fully enumerable. Use the <a href="/language/class-modifiers#sealed"><code>sealed</code> modifier</a> on a class to enable exhaustiveness checking when switching over subtypes of that class:</p> <?code-excerpt "language/lib/patterns/algebraic_datatypes.dart (algebraic-datatypes)"?> <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:#D43324">sealed</span><span style="color:#D43324"> class</span><span style="color:#0468D7"> Shape</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> Square</span><span style="color:#D43324"> implements</span><span style="color:#0468D7"> Shape</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> final</span><span style="color:#0468D7"> double</span><span style="color:#222222"> length;</span></span> <span class="line"><span style="color:#0468D7"> Square</span><span style="color:#222222">(</span><span style="color:#D43324">this</span><span style="color:#222222">.length);</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> Circle</span><span style="color:#D43324"> implements</span><span style="color:#0468D7"> Shape</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> final</span><span style="color:#0468D7"> double</span><span style="color:#222222"> radius;</span></span> <span class="line"><span style="color:#0468D7"> Circle</span><span style="color:#222222">(</span><span style="color:#D43324">this</span><span style="color:#222222">.radius);</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#0468D7">double</span><span style="color:#6200EE"> calculateArea</span><span style="color:#222222">(</span><span style="color:#0468D7">Shape</span><span style="color:#222222"> shape) => </span><span style="color:#D43324">switch</span><span style="color:#222222"> (shape) {</span></span> <span class="line"><span style="color:#0468D7"> Square</span><span style="color:#222222">(length: </span><span style="color:#D43324">var</span><span style="color:#222222"> l) => l * l,</span></span> <span class="line"><span style="color:#0468D7"> Circle</span><span style="color:#222222">(radius: </span><span style="color:#D43324">var</span><span style="color:#222222"> r) => math.pi * r * r,</span></span> <span class="line"><span style="color:#222222">};</span></span></code></pre></div></div><p>If anyone were to add a new subclass of <code>Shape</code>, this <code>switch</code> expression would be incomplete. Exhaustiveness checking would inform you of the missing subtype. This allows you to use Dart in a somewhat <a href="https://en.wikipedia.org/wiki/Algebraic_data_type">functional algebraic datatype style</a>.</p><p><a id="when"></a></p><div class="header-wrapper"><h2 id="guard-clause">Guard clause</h2><a class="heading-link" href="#guard-clause" aria-label="Link to 'Guard clause' section">#</a></div><p>To set an optional guard clause after a <code>case</code> clause, use the keyword <code>when</code>. A guard clause can follow <code>if case</code>, and both <code>switch</code> statements and expressions.</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">// Switch statement:</span></span> <span class="line"><span style="color:#D43324">switch</span><span style="color:#222222"> (something) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#222222"> somePattern </span><span style="color:#D43324">when</span><span style="color:#222222"> some || boolean || expression:</span></span> <span class="line"><span style="color:#6E6E70"> // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.</span></span> <span class="line"><span style="color:#222222"> body;</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Switch expression:</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> value = </span><span style="color:#D43324">switch</span><span style="color:#222222"> (something) {</span></span> <span class="line"><span style="color:#222222"> somePattern </span><span style="color:#D43324">when</span><span style="color:#222222"> some || boolean || expression => body,</span></span> <span class="line"><span style="color:#6E6E70"> // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// If-case statement:</span></span> <span class="line"><span style="color:#D43324">if</span><span style="color:#222222"> (something </span><span style="color:#D43324">case</span><span style="color:#222222"> somePattern </span><span style="color:#D43324">when</span><span style="color:#222222"> some || boolean || expression) {</span></span> <span class="line"><span style="color:#6E6E70"> // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Guard clause.</span></span> <span class="line"><span style="color:#222222"> body;</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>Guards evaluate an arbitrary boolean expression <em>after</em> matching. This allows you to add further constraints on whether a case body should execute. When the guard clause evaluates to false, execution proceeds to the next case rather than exiting the entire switch.</p><nav id="prev-next"><ul><li class="previous"><a href="/language/loops">&lang;  Loops</a></li><li class="next"><a href="/language/error-handling">Error handling  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.7.1. Page last updated on 2025-02-12.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/language/branches.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/language/branches/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/language/branches.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main></div><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="Dart's Medium publication"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="Dart's GitHub organization"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://bsky.app/profile/dart.dev" target="_blank" rel="noopener" title="Dart's Bluesky profile"><svg><use href="/assets/img/social/bluesky.svg#bluesky"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="Dart's X (Twitter) profile"><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></div></body></html>