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primarykey
Id
20861290
data
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0
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2013-12-31T17:46:37.610
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2014-01-01T00:45:20.387
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2014-01-01T00:45:20.387
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997251
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2
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text
Body
In Dart <ol> <li>upcasts are implicit. If <code>B</code> is a subclass of <code>A</code>, then <code>B b = a</code> (where <code>a</code> is an instance of class <code>A</code>) is warning free and silently casts <code>a</code> to <code>B</code>. In Java you would have needed to write <code>B b = (B) a</code>.</li> <li>the dynamic type is an always-present escape hatch. If <code>B</code> and <code>C</code> are not in the same hierarchy, then temporarily assigning to the dynamic type will make the cast warning-free. <pre><code>B b = someB; var tmp = b; C c = tmp; </code></pre></li> <li>One can do an explicit check using <code>is</code>. The <code>is</code> check returns true if the object is of the right type. There are also some very simple rules that propagate is-check types. For example, if <code>is</code> is used inside an <code>if</code> condition, then the corresponding branch uses this information for type-warnings. <pre><code>Object o; o.foo(); // warning: foo does not exist on Object. if (o is A) { // assuming that A contains 'foo'. o.foo(); // warning-free. } </code></pre></li> <li>One can explicitly check and throw if a type is not the expected one with <code>as</code>. This operator does not throw when the left-hand side is <code>null</code>.</li> </ol> For your example this boils down to: No explicit check (1): <pre class="lang-dart prettyprint-override"><code>void onEvent(Event event) { // Implicit upcast. PlaceChangeEvent is subclass of Event. PlaceChangeEvent pce = event; Place place = pce.place; presenterProvider.display(place); } </code></pre> With an is-check (3): <pre class="lang-dart prettyprint-override"><code>void onEvent(Event event) { if (event is PlaceChangeEvent) { // Static type system propagates type. No need for variable. Place place = event.place; presenterProvider.display(place); } else { // Note: this doesn't look like an exception, but an error. throw new Exception("Event is not a PlaceChangeEvent."); } } </code></pre> Using <code>as</code> (4): <pre class="lang-dart prettyprint-override"><code>void onEvent(Event event) { Place place = (event as PlaceChangeEvent).place; presenterProvider.display(place); } </code></pre> Alternatively, if you expect to receive a PlaceChangeEvent, you should simply change the type of the argument: <pre class="lang-dart prettyprint-override"><code>void onEvent(PlaceChangeEvent event) { Place place = event.place; presenterProvider.display(place); } </code></pre> In checked mode this will catch bad types, and in unchecked mode it will throw when accessing <code>event.place</code>. This is generally the preferred way.
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1. PODart typecasting error
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1. USFlorian Loitsch
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1. USFlorian Loitsch
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