为什么入库时间和将GUID结构进行比较，为空？ [英] Why can TimeSpan and Guid Structs be compared to null?

问题描述

我注意到，一些.NET结构可以比作空。 例如：

I've noticed that some .NET structs can be compared to null. For example:

  TimeSpan y = new TimeSpan();
        if (y == null)
            return;

编译就好了（同样具有GUID结构）。
现在我知道，stucts是值类型，而且code以上不被编译，除非有运营商的重载==这需要一个对象。但是，据我所知没有。
我看着学生与反射器，以及在MSDN上的文档。
他们两个都实现以下接口：

will compile just fine (the same with the Guid struct).
Now I know that stucts are value type and that the code above should not compile, unless there's an overload of operator == which takes an object. But, as far as I could tell there isn't.
I've looked at the class with Reflector, and also at the docs on MSDN.
The two of them do implement the following interfaces:

IComparable, IComparable<T>, IEquatable<T>

但是，试图implment相同的接口似乎并没有帮助：

but, trying to implment the same Interfaces did not seem to help:

struct XX : IComparable, IComparable<XX>, IEquatable<XX> {
    public int CompareTo(Object obj) {
        return 0;
    }
    public int CompareTo (XX other){
        return 0;
    }
    public bool Equals (XX other){
        return false;
    }
    public override bool Equals(object value){
        return false;
    }
    public static int Compare(XX t1, XX t2){
        return 0;
    }
}

我使用的是：.NET 2.0的Visual Studio 2005

I'm using: .NET 2.0 Visual Studio 2005.

有没有人有任何想法是这样做的原因是什么？ 我只是想获得一个更好的了解。这不是一个问题，因为我知道我不应该比较结构反正空。

Does anyone has any idea what's the reason for this ? I am just trying to get a better understanding. This isn't an issue as I know I shouldn't compare structs to null anyway.

推荐答案

这是 == 运营商。

在时间跨度类有相等运算符的重载：

The TimeSpan class has an overload of the equality operator:

public static bool operator ==(DateTime d1, DateTime d2)
{
     return (t1._ticks == t2._ticks);
}

这本身并不使人们有可能用空来比较，但是

This in itself doesn't make it possible to compare with null, but...

使用可空类型的到来，每个结构是隐式转换到它的可空类型，所以当你看到类似

With the arrival of nullable types, each struct is implicitly convertible to its nullable type, so when you see something like

TimeSpan y = new TimeSpan();
if (y == null)
    return;

您的不看的，这是发生了什么：

You don't see that this is happening:

TimeSpan y = new TimeSpan();
if ((Nullable<TimeSpan>)y == (Nullable<TimeSpan>)null)
    return;

空变隐式转换，但是的不是所有的 System.Object的对象的做（隐式分配？）：

Null gets the implicit conversion (implicit assignment?), but not all System.Object objects do:

TimeSpan y = new TimeSpan();
object o = null;
if (y == o) //compiler error
    return;

好了，但平等的运营商并不需要为空的论点，不是吗？

Okay, but the equality operator doesn't take nullable arguments, does it?

好了， MSDN 是帮助在这里，说：

Well, msdn is of help here, stating:

在predefined一元和二元   运营商和用户定义的任何   存在的价值类型的运营商   也可以使用由空类型。   这些运营商产生一个空值   如果[任何]操作数都为空;除此以外，   操作员使用包含的值   来计算结果。

The predefined unary and binary operators and any user-defined operators that exist for value types may also be used by nullable types. These operators produce a null value if [any of] the operands are null; otherwise, the operator uses the contained value to calculate the result.

所以，你得到有效的可为空的实现各运营商免费，有一个固定的定义的行为。而载值上面提到的是实际值非空的操作员将返回。

So you effectively get a nullable implementation for each operator for free, with a fixed defined behaviour. The "contained value" mentioned above is the actual value the non-nullable operator would return.

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