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    <p>A number of the answers here suggest benchmarking, which is a step in the right direction, but only the first step in the journey.</p> <p>My team has done a great deal of profiling and benchmarking in this area. The short version is <em>yes</em>, there are situations in which interfaces impose a small but measurable performance cost. <em>However</em> the actual cost depends on a great many factors, including how many interfaces are supported, how many of those interfaces a given reference is cast to, what the pattern of accesses are, and so on. The CLR has a great many heuristics in place designed to speed up interface access in common cases. </p> <p>If you are benchmarking one of those common cases, but your actual program falls into a less common case, then your benchmarking is <em>actively harmful</em> because it is giving you data that is misleading.</p> <p>Far better to do realistic performance measurements on <em>real</em> code. Use a profiler, write the code both ways, and see whether either way is measurably, repeatably faster in a way that is visible and relevant to the user.</p> <p>As for your reference to throwing and catching: the performance cost of throwing and catching should be irrelevant. Exceptions are by definition <em>exceptional</em>, not <em>common</em>. Furthermore, exceptions usually indicate that something is going to halt shortly; it usually doesn't matter whether something halts as fast as possible. If you are in a situation where your performance is gated by exceptions then you have bigger problems to solve: <em>stop throwing so many exceptions</em>. An exception thrown should be extremely rare.</p>
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    1. USEric Lippert
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    1. CONice answer. Couldn't agree more with regard to exceptions.
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    2. COI was surprised to see a situation with the XBox using XNA's C# where some classes implemented a seemingly reasonable number of interfaces and there was a significant penalty for casting to them. I'll be looking for more information as to why it was much more than I expected but I just wanted to drop a note here to say that sometimes it is more than a small performance problem. For me, removing some reocurring casts with a one time cast & cache saved me several milliseconds which is a lot when you consider there is only 16 of them available to maintain a 60hz display.
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