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POHow to tell which interface is returned by a method
 

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  1. POHow to tell which interface is returned by a method
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    <p>Given this code snippet that can be readily pasted into Linqpad (or slightly modified in a Visual Studio console solution):</p> <pre><code>void Main() { var cat = this.GetCat(); var dog = this.GetDog(); cat.Think(); cat.ThinkHarder(); //dog.Think(); // Does not compile. //dog.ThinkHarder(); // Does not compile. //if ([dog is returned as ISmartAnimal]) // What to put here? ((ISmartAnimal)dog).Think(); // Compiles, runs, but shouldn't. reportTypeProperties(cat); reportTypeProperties(dog); } interface IAnimal { string Name { get; set; } } interface ISmartAnimal : IAnimal { void Think(); } class Animal : IAnimal, ISmartAnimal { public string Name { get; set; } public void Think() { } } ISmartAnimal GetCat() { return new Animal(); } IAnimal GetDog() { return new Animal(); } static void reportTypeProperties(object obj) { var type = obj.GetType(); Console.WriteLine("Type: {0}", type.Name); Console.WriteLine("Is smart? {0}", obj is ISmartAnimal); } static class ext { public static void ThinkHarder(this ISmartAnimal animal) { } } </code></pre> <p>The output of <code>reportTypeProperties</code> shows that <code>dog</code>, although returned as IAnimal, "is" an ISmartAnimal. (Same for both objects)</p> <blockquote> <p>Type: Animal<br> Is smart? True</p> </blockquote> <p>This is because <code>GetType()</code> returns the concrete type of the object, not its current interface.</p> <p>My question. <strong>Is there a way to tell that <code>dog</code> is returned as IAnimal?</strong> (see pseudocode). The compiler knows (so does quickview). Suppose I had some animal object and I wanted to inspect in runtime code whether or not I can make it <code>Think()</code>.</p> <p>Background:<br> This may seem an academic exercise. And it may seem strange to have a class (Animal) implement an interface (ISmartAnimal) that you don't want to expose always. But I ask because I encountered something similar in Entity Framework. If you want you can read about it <a href="https://stackoverflow.com/questions/9025137/when-is-objectquery-really-an-iorderedqueryable">here</a>, but it diverts to EF-specific features. If you don't want to delve into that it suffices to say that it's imperative that <code>Animal</code> implement both interfaces.</p> <hr> <p>Disclaimer:<br> "Any resemblance to real animals is purely coincidental :)"</p>
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