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    <p>First off, I would not use unsafe code for this unless I had first determined (1) that the obvious code with the switch would be <strong>the slowest code in the entire program</strong> and causing significant, user-observable slowdowns, and that (2) going to unsafe code fixes the performance problem.</p> <p>Second, if I were to use unsafe code, it is extraordinarily dangerous to make assumptions about structure packing. The CLR is allowed broad lattitude in how it chooses to pack structures. <strong>If you are going to do this dangerous thing then you should use the struct layout attribute to ensure that the floats are exactly where you need them to be.</strong></p> <p>Third, what stops a buggy caller from passing a negative index, or a too-large index?</p> <p>Fourth:</p> <blockquote> <p>Does fixed create redundant overhead here? </p> </blockquote> <p>I don't know what "redundant overhead" means. "fixed" makes the jitter tell the garbage collector "do not move this thing because I need to do pointer arithmetic on it". You are fixing for a short period, which is ideal; fixing for a long time makes it more likely that a collection will get messed up because the pinned storage could not be moved.</p> <p>Fifth:</p> <blockquote> <p>Or maybe this struct is already fixed, and therefore fixed has no effect here and is only needed to satisfy the compiler?</p> </blockquote> <p>Maybe! Maybe the variable referred to by "this" is already a fixed variable. <strong>Maybe it isn't.</strong> How is the <em>compiler</em> supposed to know whether the "this" of the struct is a ref to fixed storage or not? We have to assume the worst, so you are required to fix it.</p>
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    1. POGetting pointer to struct inside itself (unsafe context)
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    1. USEric Lippert
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    1. COBut the `struct` in the question has a sequential layout in managed memory and contiguously packed types by default, doesn't it?
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    2. CO@Eric yeah I've got the same question - I read somewhere that structs have strict sequential layout policy. Anyway my struct already explicitly defines the layout, so this is not a problem.
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      1. USAlexander Shukaev
    3. CO@ordag: I don't know -- does it? The C# specification states "The order in which members are packed into a struct is unspecified. For alignment purposes, there may be unnamed padding at the beginning of a struct, within a struct, and at the end of the struct." You are given *no guarantee whatsoever* that structures are packed in any particular way. If some implementation that you *happen* to be using just *happens* to pack structures in a way that you *happen* to like, that's your good luck. How far do you want to trust that luck?
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