如何知道项目在枚举中？ [英] How can I know items is in the enum?

问题描述

在此问题中，我使用xor运算符枚举与 [Flags] 属性如下：

In this question, I use xor operator between enum with [Flags] attribute as following:

[Flags]
enum QueryFlag
{
  None = 0x1,
  ByCustomer = 0x2,
  ByProduct = 0x4,
  ByDate = 0x8
}
QueryFlag flags = QueryFlag.ByCustomer | QueryFlag.ByProduct;

要添加QueryFlag，当然我们应该使用 | operator。

To add an QueryFlag, of course we should use | operator.

flags |= QueryFlag.ByDate;

要删除一个，我有一种不同的方式与丹涛的回答。我正在使用：

To remove one, I have a different way with Dan Tao's answer. I'm using:

flags ^= QueryFlag.ByProduct;

他正在使用：

flags &= ~QueryFlag.ByProduct;

显然他的答案是正确和容易理解的。我以为我犯了一个错误。但是经过深思，我得到了：

Obviously his answer is correct and easy to understand. I thought I made a mistake. But after a deep thought I got:

a,b         a^b         a&(~b)
0,0          0           0
0,1          1           0   //the difference
1,0          1           1
1,1          0           0

现在我知道我的错误。 ^ 在您尝试删除不存在的项目时出错。

And now I knew my mistake. ^ is wrong when you try to remove one item which doesn't exist.

QueryFlag q = QueryFlag.ByCustomer | QueryFlag.ByDate;
//try to remove QueryFlag.ByProduct which doesn't exist in q
q ^ QueryFlag.ByProduct    //equals to add ByProduct to q, wrong!
q & (~QueryFlag.ByProduct) // q isn't changed, remain the original value. correct!

但这里有另一个问题：我如何知道如果 q 包含一个项目？基于Dan Tao的答案我写了一个扩展名：

But here I got another question: how can I know if q contains one item? Base on Dan Tao's answer I wrote an extension:

public static bool Contains(this QueryFlag flags, QueryFlag flag)
{
   return (flags & (~flag)) != flags;
}

也就是说，如果标志在从标志中移除标志后没有更改，那么我们知道国旗不在国旗！以下情况似乎是正确的：

That is, if flags is not changed after removing flag from flags, we know flag is not in flags! It seems correct when:

(QueryFlag.ByProduct | QueryFlag.ByDate).Contains(QueryFlag.None)   //false
(QueryFlag.ByProduct | QueryFlag.ByDate).Contains(QueryFlag.ByDate)  //true

但实际上：

(QueryFlag.ByProduct | QueryFlag.ByDate).Contains(QueryFlag.ByDate | QueryFlag.ByCustomer) //true, but I suppose it's false

我知道为什么它是假的原因，我该如何改进？这是第一个问题。
第二个：我想使 .Contains 通用到更多枚举与 [Flags] 属性

I know the reason why it's false, how can I improve it? It's the first question. The second: I want to make the .Contains generic to more enum with [Flags] attribute.

public static bool Contains<T>(this T flags, T flag) where T : Enum//with [Flags]
{
    return (flags & (~flag)) != flags;
}

可能无法限制T标记的属性。但即使我删除了这个约束，我得到一个编译错误，说 operator〜不能应用于类型T 。为什么和如何解决？

Probably it's impossible to constrain T with attribute marked. But even I remove this constraint I get a compile error which says operator ~ can't be applied to type T. Why and how to resolve?

推荐答案

您的错误在于这种方法：

Your error lies in this method:

public static bool Contains(this QueryFlag flags, QueryFlag flag)
{
   return (flags & (~flag)) != flags;
}

每当 flags 具有标志中包含的标志的任何（即至少一个），但我想你想要 all 。

This returns true whenever flags has any (i.e. at least one) of the flags contained in flag, but I think you want all.

它应该是：

public static bool Contains(this QueryFlag flags, QueryFlag flag)
{
   return (flags & flag) == flag;
}

或者，您只需使用 Enum.HasFlag ）这样做。例如：

Alternatively, you can just use Enum.HasFlag() which does exactly this. For example:

QueryFlag qf = ...;
if (qf.HasFlag(QueryFlag.ByCustomer))
    // ...

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