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copied!<p>There is no way, in general, to determine if an object has a value for a given key. An instance may decide to return a value for an otherwise undefined key from its <code>-valueForUndefinedKey:</code> method. Or it may let the default implementation throw an exception. Modern Objective-C (2.0) objects <em>often</em> declare relevant properties in their public API. For these objects, you can use the Objective-C runtime's <code>class_copyPropertyList</code> or <code>protocol_copyPropertyList</code> to get a list of the available properties. You can also emulate the KVC search list, testing via <code>repondsToSelector:</code> if the instance responds to an appropriate getter method. Finally you could use the runtime's <code>class_copyIvarList</code> to check the ivars for an appropriate ivar. This is a <em>lot</em> of work that you <strong>shouldn't be doing</strong>. It won't guarantee that you know whether an object instance will raise an exception when sent a <code>valueForKey:</code> message and it indicates a bigger issue...</p> <p>If you have a collection of objects that must all provide a value for a given key, but some don't, you have a design problem. You should define an appropriate interface (a <code>@protocol</code> in Objective-C) that declares the needed properties. You can then test for conformance to this protocol, or make use of the compiler to do some compile-time type checking for you to make sure instance conform to the protocol (if you're passing around single instances).</p>

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