“bool"和“bool"有什么区别?和“布尔?"?

The ? symbol after a type is only a shortcut to the Nullable type, bool? is equivalent to Nullable<bool>.

bool 是 值类型, 这意味着它不能是 null，所以 Nullable 类型基本上允许你包装值类型，并且能够将 null 分配给它们.

bool is a value type, this means that it cannot be null, so the Nullable type basically allows you to wrap value types, and being able to assign null to them.

bool? 可以包含三个不同的值:true、false 和 null.

bool? can contain three different values: true, false and null.

此外，没有为 bool?

仅定义了逻辑 AND(包括 OR)运算符，它们的行为如下:

Only the logical AND, inclusive OR, operators are defined and they behave like this:

x        y      x & y   x | y   
true    true    true    true
true    false   false   true
true    null    null    true
false   true    false   true
false   false   false   false
false   null    false   null
null    true    null    true
null    false   false   null
null    null    null    null

Nullable 类型基本上是一个泛型结构，具有以下公共属性:

The Nullable type is basically a generic struct, that has the following public properties:

public struct Nullable<T> where T: struct
{
    public bool HasValue { get; }
    public T Value { get; }
}

HasValue属性表示当前对象是否有值，Value属性会获取对象的当前值，如果HasValue为false则抛出 InvalidOperationException.

The HasValue property indicates whether the current object has a value, and the Value property will get the current value of the object, or if HasValue is false, it will throw an InvalidOperationException.

现在你一定很疑惑，Nullable是一个struct，一个不能为null的值类型，那么为什么下面的语句是有效的呢?

Now you must be wondering something, Nullable is a struct, a value type that cannot be null, so why the following statement is valid?

int? a = null;

该示例将编译为:

.locals init (valuetype [mscorlib]System.Nullable`1<int32> V_0)
IL_0000:  ldloca.s   V_0
IL_0002:  initobj    valuetype [mscorlib]System.Nullable`1<int32>

调用 initobj，它将指定地址处的值类型的每个字段初始化为空引用或相应原始类型的 0.

A call to initobj, which initializes each field of the value type at a specified address to a null reference or a 0 of the appropriate primitive type.

就是这样，这里发生的是默认的 struct初始化.

That's it, what's happening here is the default struct initialization.

int? a = null;

相当于:

Nullable<int> a = new Nullable<int>();

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