C＃编译器不会优化不必要的转换 [英] C# compiler doesn’t optimize unnecessary casts

问题描述

几天后，在这里写下这个问题的答案在溢出我有一个惊讶的C＃编译器，谁没有做我期望它做的。查看以下代码片段：

A few days back, while writing an answer for this question here on overflow I got a bit surprised by the C# compiler, who wasn’t doing what I expected it to do. Look at the following to code snippets:

第一个：

object[] array = new object[1];

for (int i = 0; i < 100000; i++)
{
    ICollection<object> col = (ICollection<object>)array;
    col.Contains(null);
}

其次：

object[] array = new object[1];

for (int i = 0; i < 100000; i++)
{
    ICollection<object> col = array;
    col.Contains(null);
}

两个代码段之间的唯一区别是转换为ICollection< object >。因为object []显式地实现了ICollection< object>接口，所以我期望这两个片段编译到同一个IL，因此是相同的。然而，当对他们进行性能测试时，我注意到后者的速度是前者的6倍。

The only difference in code between the two snippets is the cast to ICollection<object>. Because object[] implements the ICollection<object> interface explicitly, I expected the two snippets to compile down to the same IL and be therefore identical. However, when running performance tests on them, I noticed the latter to be about 6 times as fast as the former.

在比较两个片段的IL之后，我注意到除了第一个片段中的 castclass IL指令，两种方法都是相同的。

After comparing the IL from both snippets, I noticed the both methods were identical, except for a castclass IL instruction in the first snippet.

我惊讶于这一点我现在想知道为什么C＃在这里聪明。事情从来没有像它看起来那么简单，所以为什么C＃编译器有点幼稚？

Surprised by this I now wonder why the C# compiler isn’t ‘smart’ here. Things are never as simple as it seems, so why is the C# compiler a bit naïve here?

推荐答案

在优化器中发现了一个小错误。有各种特殊情况的代码在那里的数组。感谢您提请我注意。

My guess is that you have discovered a minor bug in the optimizer. There is all kinds of special-case code in there for arrays. Thanks for bringing it to my attention.

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