UselessIsBeforeAs.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>The C# <code>as</code> operator is equivalent to testing if an object is of a specific type with 
<code>is</code> and then performing a cast or returning null depending on the result. Performing an 
<code>is</code> check before calling <code>as</code> is completely redundant because an <code>is
</code> check is performed twice.</p>

</overview>
<recommendation>
<p>Just use <code>as</code> on its own and then perform a null check on the result to determine if 
the cast was successful. Note in passing that it is no better to replace the 'as' with a cast, 
since that also does a type test internally.</p>

</recommendation>
<example>
<p>In this example two type checks are performed for <code>a</code> against <code>Rectangle</code>.
</p>
<sample src="UselessIsBeforeAs.cs" />

<p>Here is the same function performed more efficiently by omitting the extra type check.</p>
<sample src="UselessIsBeforeAsFix.cs" />

</example>
<references>

  <li>MSDN: <a href="http://msdn.microsoft.com/en-us/library/scekt9xw(v=vs.71).aspx">is</a></li>
  <li>MSDN: <a href="http://msdn.microsoft.com/en-us/library/cscsdfbt(v=vs.71).aspx">as</a></li>

</references>
</qhelp>