显式空核对与空聚合运算符的编译器评估？ [英] Compiler evaluation of explicit null-check vs. null-coalescing operator?

问题描述

请考虑以下代码，它使用两种稍微不同的方法来检查 _instance ，并在尚未设置时分配。

  class InstantiationTest 
 {
 private Object _instance; 
 
 public void Method1（）{
 if（_instance == null）{
 _instance = new Object（）; 
} 
} 
 
 public void Method2（）{
 _instance = _instance？ new Object（）; VS或Resharper会继续强调我的显式空检查，但是我们可以看到，并提示我使用null聚结运算符重构。 
 
 
 
我想知道编译器是否足够智能以检测 Method2（）中的情况，其中 _instance 重新分配给自身（实际上是 nop ？）并将 Method2（）重写为 Method1（）。 
 
 
 
我看到的并不是这样：
 方法1：
 IL_0000：ldarg.0 
 IL_0001：ldfld UserQuery + Test._instance 
 IL_0006：brtrue.s IL_0013 
 IL_0008：ldarg.0 
 IL_0009：newobj System .Object..ctor 
 IL_000E：stfld UserQuery + Test._instance 
 IL_0013：ret 
  
与：
  Test.Method2：
 IL_0000：ldarg.0 
 IL_0001：ldarg。 0 
 IL_0002：ldfld UserQuery + Test._instance 
 IL_0007：dup 
 IL_0008：brtrue.s IL_0010 
 IL_000A：pop 
 IL_000B：newobj System.Object..ctor 
 IL_0010：stfld UserQuery + Test._instance 
 IL_0015：ret 
  
是为什么？ 
 
 
 
在编译器级别实现是很棘手的，太麻烦了，值得付出努力，还是我错过了什么？ 
解决方案
一般来说，C＃编译器对IL进行非常少的优化，直到JIT，用于特定架构。所以它只是没有在编译器中实现，因为这将需要时间远离其他的东西。
 
Consider the following code, which uses two slightly different methods to check _instance and assign it when not already set.
class InstantiationTest
{
    private Object _instance;

    public void Method1() {
        if(_instance == null) {
            _instance = new Object();
        }
    }

    public void Method2() {
        _instance = _instance ?? new Object();
    }
}
Either VS or Resharper keeps underlining my explicit null checks, and prompting me to refactor using the null-coalescing operator. 

I wondered whether the compiler is smart enough to detect the case in Method2() where _instance is reassigned to itself (effectively a nop?) and rewrite Method2() into Method1(). 

I see this is not actually the case:
Test.Method1:
IL_0000:  ldarg.0     
IL_0001:  ldfld       UserQuery+Test._instance
IL_0006:  brtrue.s    IL_0013
IL_0008:  ldarg.0     
IL_0009:  newobj      System.Object..ctor
IL_000E:  stfld       UserQuery+Test._instance
IL_0013:  ret   
versus:      
Test.Method2:
IL_0000:  ldarg.0     
IL_0001:  ldarg.0     
IL_0002:  ldfld       UserQuery+Test._instance
IL_0007:  dup         
IL_0008:  brtrue.s    IL_0010
IL_000A:  pop         
IL_000B:  newobj      System.Object..ctor
IL_0010:  stfld       UserQuery+Test._instance
IL_0015:  ret      
My question is why? 

Is it tricky to implement at the compiler level, too trivial to be worth the effort of implementation, or something I'm missing?  
解决方案
Generally, the C# compiler does very little optimizing of the IL, leaving that up to the JIT, which optimizes things much better for a specific architecture. So it's simply not been implemented within the compiler, as that would take time away from other things.

                        
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