在集合类型上使用支撑式初始化程序会设置初始容量吗？

问题描述

是否在集合类型上使用支撑式初始化程序设置了它的容量，还是还需要指定它的容量？

Does using the braced initializer on a collection type set it's capacity or do you still need to specify it?

也就是说，是：

var list = new List<string>(){ "One", "Two" };

结果与以下相同：

var list = new List<string>(2){ "One", "Two" };

推荐答案

对象初始化程序仅调用 Add 。

Object initializer simply calls Add for each item.

var list = new List<string>{ "One", "Two", "Three" };

如您所见，在这种情况下，无参数构造函数称为：

As you can see, in this case parameterless constructor is called:

L_0000: nop 
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor()
L_0006: stloc.1 
L_0007: ldloc.1 
L_0008: ldstr "One"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_0012: nop 
L_0013: ldloc.1 
L_0014: ldstr "Two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_001e: nop 
L_001f: ldloc.1 
L_0020: ldstr "Three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_002a: nop 
L_002b: ldloc.1 

因此，您应该手动设置容量：

So, you should set capacity manually:

var list = new List<string>(5){ "One", "Two", "Three" };

编译为：

L_0000: nop 
L_0001: ldc.i4.5 
L_0002: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor(int32)
// rest is same

因此，算法非常明显-它调用构造函数您指定了（默认情况下为无参数），然后为每个项目调用添加。

So, algorithm is pretty obvious - it calls constructor which you specified (parameterless by default) and then calls Add for each item.

注意：我知道，该默认值 List< T> 的容量为4，我验证了如果我们在初始化程序中传递4个以上的项目（例如，编译器根据项目数确定要调用哪个构造函数）会发生什么情况，但是结果是相同的-默认情况下将调用无参数构造函数。

NOTE: I know, that default capacity is 4 for List<T> and I verified what happens if we pass more than 4 items in initializer (e.g. maybe compiler determines which constructor to call depending on items count) but result is same - parameterless constructor is called by default.

我认为集合初始值设定项的目的是创建小的集合（1-8个项目），因此会有一点性能影响（如果将8个项目传递给初始值设定项，则仅调整大小）。没有人期望您将对100个项目使用就地初始化。而且，如果要执行此操作，则应使用适当的collection构造函数。

I think purpose of collection initializers is creating small collections (1 - 8 items), thus there will be a little performance impact (only one resize if you will pass 8 items into initializer). Nobody expects you will use in-place initialization with 100 items. And if you are going to do that, you should use appropriate constructor of collection.

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