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copied!<p>The answer below was written years ago and updated over time. As of C# 7, you can use pattern matching:</p> <pre><code>if (animal is Dog dog) { // Use dog here } </code></pre> <p>Note that <code>dog</code> is still in scope after the <code>if</code> statement, but isn't definitely assigned.</p> <hr> <p>No, there isn't. It's more idiomatic to write this though:</p> <pre><code>Dog dog = animal as Dog; if (dog != null) { // Use dog } </code></pre> <p>Given that "as followed by if" is almost <em>always</em> used this way, it might make more sense for there to be an operator which performs both parts in one go. This isn't currently in C# 6, but may be part of C# 7, if the <a href="https://github.com/dotnet/roslyn/issues/206" rel="noreferrer">pattern matching proposal</a> is implemented.</p> <p>The problem is that you can't <em>declare</em> a variable in the condition part of an <code>if</code> statement<sup>1</sup>. The closest approach I can think of is this:</p> <pre><code>// EVIL EVIL EVIL. DO NOT USE. for (Dog dog = animal as Dog; dog != null; dog = null) { ... } </code></pre> <p>That's just <em>nasty</em>... (I've just tried it, and it does work. But please, please don't do this. Oh, and you can declare <code>dog</code> using <code>var</code> of course.)</p> <p>Of course you could write an extension method:</p> <pre><code>public static void AsIf<T>(this object value, Action<T> action) where T : class { T t = value as T; if (t != null) { action(t); } } </code></pre> <p>Then call it with:</p> <pre><code>animal.AsIf<Dog>(dog => { // Use dog in here }); </code></pre> <p>Alternatively, you could combine the two:</p> <pre><code>public static void AsIf<T>(this object value, Action<T> action) where T : class { // EVIL EVIL EVIL for (var t = value as T; t != null; t = null) { action(t); } } </code></pre> <p>You can also use an extension method without a lambda expression in a cleaner way than the for loop:</p> <pre><code>public static IEnumerable<T> AsOrEmpty(this object value) { T t = value as T; if (t != null) { yield return t; } } </code></pre> <p>Then:</p> <pre><code>foreach (Dog dog in animal.AsOrEmpty<Dog>()) { // use dog } </code></pre> <hr> <p><sup>1</sup> You can <em>assign</em> values in <code>if</code> statements, although I rarely do so. That's not the same as declaring variables though. It's not <em>terribly</em> unusual for me to do it in a <code>while</code> though when reading streams of data. For example:</p> <pre><code>string line; while ((line = reader.ReadLine()) != null) { ... } </code></pre> <p>These days I normally prefer to use a wrapper which lets me use <code>foreach (string line in ...)</code> but I view the above as a pretty idiomatic pattern. It's <em>usually</em> not nice to have side-effects within a condition, but the alternatives usually involve code duplication, and when you know this pattern it's easy to get right.</p>

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