如何在.NET中的引擎盖下实现的可空类型? [英] how are nullable types implemented under the hood in .net?
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问题描述
在我们自己的乔恩斯基特的 C#深入,他讨论了3种方式来模拟一个'空'的值类型:
In our own Jon Skeet's C# in depth, he discusses the 3 ways to simulate a 'null' for value types:
- 魔法值(例如最早可能日期时间取为'空')
- 引用类型的包装
- 布尔标志
要提及的是空类型使用第三种方法。究竟如何做到可空类型的发动机罩下工作吗?
It is mentioned that nullable types use the third method. How exactly do nullable types work under the hood?
推荐答案
最后,他们都只是一个布尔标志的通用结构 - 除非有特殊拳击规则。因为结构是(默认)初始化为零,所述布尔默认为假(没有值):
Ultimately, they are just a generic struct with a bool flag - except with special boxing rules. Because structs are (by default) initialized to zero, the bool defaults to false (no value):
public struct Nullable<T> where T : struct {
private readonly T value;
private readonly bool hasValue;
public Nullable(T value) {
this.value = value;
hasValue = true;
}
public T Value {
get {
if(!hasValue) throw some exception ;-p
return value;
}
}
public T GetValueOrDefault() { return value; }
public bool HasValue {get {return hasValue;}}
public static explicit operator T(Nullable<T> value) {
return value.Value; }
public static implicit operator Nullable<T>(T value) {
return new Nullable<T>(value); }
}
额外差异,虽然:
Extra differences, though:
- 特殊拳击规则(您无法正常做到这一点)
- 特殊C#的规则比较空等
- 解禁运营商在C#(和.NET通过
EqualityComparer&LT; T&GT;
,的Comparer&LT; T&GT;
等) - 特殊规则(以prevent
可空&LT;可空&LT; T&GT;&GT;
)
泛型类型约束
- special boxing rules (you can't normally do this)
- special C# rules for comparing to null etc
- "lifted" operators in C# (and in .NET via
EqualityComparer<T>
,Comparer<T>
etc) - special rules on generic type constraints (to prevent
Nullable<Nullable<T>>
)
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