为什么通用方法的T constaint：类结果在拳击？ [英] Why does generic method with constaint of T: class result in boxing?

问题描述

任何人任何想法，为什么这限制了T可类泛型方法将有拳击的指令在生成MSIL code？

Anyone any idea why a generic method which constrains T to class would have boxing instructions in the generates MSIL code?

我被这非常惊讶，因为肯定，因为T被被限制为引用类型生成code应该不需要执行任何拳击。

I was quite surprised by this since surely since T is being constrained to a reference type the generated code should not need to perform any boxing.

下面是C＃code：

protected void SetRefProperty<T>(ref T propertyBackingField, T newValue) where T : class
{
    bool isDifferent = false;

    // for reference types, we use a simple reference equality check to determine
    // whether the values are 'equal'.  We do not use an equality comparer as these are often
    // unreliable indicators of equality, AND because value equivalence does NOT indicate
    // that we should share a reference type since it may be a mutable.

    if (propertyBackingField != newValue)
    {
        isDifferent = true;
    }
}

下面是产生IL：

.method family hidebysig instance void SetRefProperty<class T>(!!T& propertyBackingField, !!T newValue) cil managed
{
    .maxstack 2
    .locals init (
        [0] bool isDifferent,
        [1] bool CS$4$0000)
    L_0000: nop 
    L_0001: ldc.i4.0 
    L_0002: stloc.0 
    L_0003: ldarg.1 
    L_0004: ldobj !!T
    L_0009: box !!T
    L_000e: ldarg.2 
    L_000f: box !!T
    L_0014: ceq 
    L_0016: stloc.1 
    L_0017: ldloc.1 
    L_0018: brtrue.s L_001e
    L_001a: nop 
    L_001b: ldc.i4.1 
    L_001c: stloc.0 
    L_001d: nop 
    L_001e: ret 
}

注意在中!!牛逼的说明。

任何人，为什么这是正在发生什么想法？

Anyone any idea why this is being generated?

任何人任何想法如何避免这种情况？

Anyone any ideas how to avoid this?

谢谢， 菲尔

推荐答案

您不必担心任何性能劣化从中指令，因为如果它的参数是引用类型，在中指令不执行任何操作。虽然它仍然奇怪的是，中指令，甚至被创建（也许lazyiness /很容易在code代的设计？）。

You don't have to worry about any performance-degradations from the box instruction because if its argument is a reference type, the box instruction does nothing. Though it's still strange that the box instruction has even been created (maybe lazyiness/easier design at code generation?).

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