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Error handling | Dart
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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="#exceptions">Exceptions</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#throw">Throw</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#catch">Catch</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#finally">Finally</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#assert">Assert</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/language/branches">⟨ Branches</a></li><li class="next"><a href="/language/classes">Classes ⟩</a></li></ul></nav><div id="site-content-title"><h1>Error handling</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="#exceptions">Exceptions</a><ul><li class="toc-entry"><a href="#throw">Throw</a></li><li class="toc-entry"><a href="#catch">Catch</a></li><li class="toc-entry"><a href="#finally">Finally</a></li></ul></li><li class="toc-entry"><a href="#assert">Assert</a></li></ul></div><div class="header-wrapper"><h2 id="exceptions">Exceptions</h2><a class="heading-link" href="#exceptions" aria-label="Link to 'Exceptions' section">#</a></div><p>Your Dart code can throw and catch exceptions. Exceptions are errors indicating that something unexpected happened. If the exception isn't caught, the <a href="/language/concurrency#isolates">isolate</a> that raised the exception is suspended, and typically the isolate and its program are terminated.</p><p>In contrast to Java, all of Dart's exceptions are unchecked exceptions. Methods don't declare which exceptions they might throw, and you aren't required to catch any exceptions.</p><p>Dart provides <a href="https://api.dart.dev/dart-core/Exception-class.html"><code>Exception</code></a> and <a href="https://api.dart.dev/dart-core/Error-class.html"><code>Error</code></a> types, as well as numerous predefined subtypes. You can, of course, define your own exceptions. However, Dart programs can throw any non-null object—not just Exception and Error objects—as an exception.</p><div class="header-wrapper"><h3 id="throw">Throw</h3><a class="heading-link" href="#throw" aria-label="Link to 'Throw' section">#</a></div><p>Here's an example of throwing, or <em>raising</em>, an exception:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (throw-FormatException)"?> <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">throw</span><span style="color:#0468D7"> FormatException</span><span style="color:#222222">(</span><span style="color:#11796D">'Expected at least 1 section'</span><span style="color:#222222">);</span></span></code></pre></div></div><p>You can also throw arbitrary objects:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (out-of-llamas)"?> <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">throw</span><span style="color:#11796D"> 'Out of llamas!'</span><span style="color:#222222">;</span></span></code></pre></div></div><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>Production-quality code usually throws types that implement <a href="https://api.dart.dev/dart-core/Error-class.html"><code>Error</code></a> or <a href="https://api.dart.dev/dart-core/Exception-class.html"><code>Exception</code></a>.</p></div></aside><p>Because throwing an exception is an expression, you can throw exceptions in => statements, as well as anywhere else that allows expressions:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (throw-is-an-expression)"?> <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">void</span><span style="color:#6200EE"> distanceTo</span><span style="color:#222222">(</span><span style="color:#0468D7">Point</span><span style="color:#222222"> other) => </span><span style="color:#D43324">throw</span><span style="color:#0468D7"> UnimplementedError</span><span style="color:#222222">();</span></span></code></pre></div></div><div class="header-wrapper"><h3 id="catch">Catch</h3><a class="heading-link" href="#catch" aria-label="Link to 'Catch' section">#</a></div><p>Catching, or capturing, an exception stops the exception from propagating (unless you rethrow the exception). Catching an exception gives you a chance to handle it:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (try)"?> <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">try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> breedMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">on</span><span style="color:#0468D7"> OutOfLlamasException</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> buyMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>To handle code that can throw more than one type of exception, you can specify multiple catch clauses. The first catch clause that matches the thrown object's type handles the exception. If the catch clause does not specify a type, that clause can handle any type of thrown object:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (try-catch)"?> <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">try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> breedMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">on</span><span style="color:#0468D7"> OutOfLlamasException</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // A specific exception</span></span> <span class="line"><span style="color:#6200EE"> buyMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">on</span><span style="color:#0468D7"> Exception</span><span style="color:#D43324"> catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6E6E70"> // Anything else that is an exception</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Unknown exception: </span><span style="color:#11796D">$</span><span style="color:#222222">e</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6E6E70"> // No specified type, handles all</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Something really unknown: </span><span style="color:#11796D">$</span><span style="color:#222222">e</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>As the preceding code shows, you can use either <code>on</code> or <code>catch</code> or both. Use <code>on</code> when you need to specify the exception type. Use <code>catch</code> when your exception handler needs the exception object.</p><p>You can specify one or two parameters to <code>catch()</code>. The first is the exception that was thrown, and the second is the stack trace (a <a href="https://api.dart.dev/dart-core/StackTrace-class.html"><code>StackTrace</code></a> object).</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (try-catch-2)" replace="/\(e.*?\)/[!$&!]/g"?> <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">try</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 style="color:#D43324">on</span><span style="color:#0468D7"> Exception</span><span style="color:#D43324"> catch</span><span style="color:#222222"> </span><mark class="highlight"><span style="color:#222222">(e)</span></mark><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Exception details:</span><span style="color:#222222">\n</span><span style="color:#11796D"> $</span><span style="color:#222222">e</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">catch</span><span style="color:#222222"> </span><mark class="highlight"><span style="color:#222222">(e, s)</span></mark><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Exception details:</span><span style="color:#222222">\n</span><span style="color:#11796D"> $</span><span style="color:#222222">e</span><span style="color:#11796D">'</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">'Stack trace:</span><span style="color:#222222">\n</span><span style="color:#11796D"> $</span><span style="color:#222222">s</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>To partially handle an exception, while allowing it to propagate, use the <code>rethrow</code> keyword.</p> <?code-excerpt "misc/test/language_tour/exceptions_test.dart (rethrow)" replace="/rethrow;/[!$&!]/g"?> <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">void</span><span style="color:#6200EE"> misbehave</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#D43324"> try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> dynamic</span><span style="color:#222222"> foo = </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">(foo++); </span><span style="color:#6E6E70">// Runtime error</span></span> <span class="line"><span style="color:#222222"> } </span><span style="color:#D43324">catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'misbehave() partially handled </span><span style="color:#11796D">${</span><span style="color:#222222">e</span><span style="color:#11796D">.</span><span style="color:#222222">runtimeType</span><span style="color:#11796D">}</span><span style="color:#11796D">.'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">rethrow</span><span style="color:#222222">;</span></mark><span style="color:#222222"> </span><span style="color:#6E6E70">// Allow callers to see the exception.</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> main</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#D43324"> try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> misbehave</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222"> } </span><span style="color:#D43324">catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'main() finished handling </span><span style="color:#11796D">${</span><span style="color:#222222">e</span><span style="color:#11796D">.</span><span style="color:#222222">runtimeType</span><span style="color:#11796D">}</span><span style="color:#11796D">.'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><div class="header-wrapper"><h3 id="finally">Finally</h3><a class="heading-link" href="#finally" aria-label="Link to 'Finally' section">#</a></div><p>To ensure that some code runs whether or not an exception is thrown, use a <code>finally</code> clause. If no <code>catch</code> clause matches the exception, the exception is propagated after the <code>finally</code> clause runs:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (finally)"?> <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">try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> breedMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">finally</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // Always clean up, even if an exception is thrown.</span></span> <span class="line"><span style="color:#6200EE"> cleanLlamaStalls</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The <code>finally</code> clause runs after any matching <code>catch</code> clauses:</p> <?code-excerpt "misc/lib/language_tour/exceptions.dart (try-catch-finally)"?> <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">try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> breedMoreLlamas</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'Error: </span><span style="color:#11796D">$</span><span style="color:#222222">e</span><span style="color:#11796D">'</span><span style="color:#222222">); </span><span style="color:#6E6E70">// Handle the exception first.</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">finally</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> cleanLlamaStalls</span><span style="color:#222222">(); </span><span style="color:#6E6E70">// Then clean up.</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>To learn more, check out the <a href="/libraries/dart-core#exceptions">core library exception docs</a>.</p><div class="header-wrapper"><h2 id="assert">Assert</h2><a class="heading-link" href="#assert" aria-label="Link to 'Assert' section">#</a></div><p>During development, use an assert statement— <code>assert(<condition>, <optionalMessage>);</code> —to disrupt normal execution if a boolean condition is false.</p> <?code-excerpt "misc/test/language_tour/control_flow_test.dart (assert)"?> <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">// Make sure the variable has a non-null value.</span></span> <span class="line"><span style="color:#D43324">assert</span><span style="color:#222222">(text != </span><span style="color:#11796D">null</span><span style="color:#222222">);</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Make sure the value is less than 100.</span></span> <span class="line"><span style="color:#D43324">assert</span><span style="color:#222222">(number < </span><span style="color:#11796D">100</span><span style="color:#222222">);</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Make sure this is an https URL.</span></span> <span class="line"><span style="color:#D43324">assert</span><span style="color:#222222">(urlString.</span><span style="color:#6200EE">startsWith</span><span style="color:#222222">(</span><span style="color:#11796D">'https'</span><span style="color:#222222">));</span></span></code></pre></div></div><p>To attach a message to an assertion, add a string as the second argument to <code>assert</code> (optionally with a <a href="/language/collections#trailing-comma">trailing comma</a>):</p> <?code-excerpt "misc/test/language_tour/control_flow_test.dart (assert-with-message)"?> <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">assert</span><span style="color:#222222">(urlString.</span><span style="color:#6200EE">startsWith</span><span style="color:#222222">(</span><span style="color:#11796D">'https'</span><span style="color:#222222">),</span></span> <span class="line"><span style="color:#11796D"> 'URL (</span><span style="color:#11796D">$</span><span style="color:#222222">urlString</span><span style="color:#11796D">) should start with "https".'</span><span style="color:#222222">);</span></span></code></pre></div></div><p>The first argument to <code>assert</code> can be any expression that resolves to a boolean value. If the expression's value is true, the assertion succeeds and execution continues. If it's false, the assertion fails and an exception (an <a href="https://api.dart.dev/dart-core/AssertionError-class.html"><code>AssertionError</code></a>) is thrown.</p><p>When exactly do assertions work? That depends on the tools and framework you're using:</p><ul><li>Flutter enables assertions in <a href="https://docs.flutter.dev/testing/debugging#debug-mode-assertions">debug mode.</a></li><li>Development-only tools such as <a href="/tools/webdev#serve"><code>webdev serve</code></a> typically enable assertions by default.</li><li>Some tools, such as <a href="/tools/dart-run"><code>dart run</code></a> and <a href="/tools/dart-compile#js"><code>dart compile js</code></a> support assertions through a command-line flag: <code>--enable-asserts</code>.</li></ul><p>In production code, assertions are ignored, and the arguments to <code>assert</code> aren't evaluated.</p><nav id="subnav"><ul><li class="previous"><a href="/language/branches">⟨ Branches</a></li><li class="next"><a href="/language/classes">Classes ⟩</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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