c# - 无法实习可为空的日期时间

Question

我正在尝试实习一些传递的对象值，这些值会长时间保存在内存中。其中一些对象是 Nullable 值类型。我无法正确地实习 Nullable 值，我认为可能会发生某种我不理解的“有用”自动装箱。这是我应该通过的单元测试(假设 Nullalbes 表现得像对象)但不是:

[Test]
public void InternNullableType()
{
    DateTime? one = new DateTime(2010, 2, 3, 4, 5, 6, DateTimeKind.Utc);
    DateTime? two = new DateTime(2010, 2, 3, 4, 5, 6, DateTimeKind.Utc);

    // should be equal, but not reference equal
    Assert.False(ReferenceEquals(one, two));
    Assert.True(one.Equals(two));

    // create an interning dictionary
    Dictionary<DateTime?, DateTime?> intern = new Dictionary<DateTime?, DateTime?>();
    intern[one] = one; // add 'one', this will be the value we hand out
    two = intern[two]; // intern the value of two

    // values should be equal, and reference qual
    Assert.True(one.Equals(two));     

    // this fails when it passes for objects       
    Assert.True(ReferenceEquals(one, two));
}

这是怎么回事？

Answer 1

可空类型是结构，而不是对象。它们是可以分配为空的特殊结构。所以实习不会像处理字符串那样工作，因为 string是引用类型。

当您检索可为 null 的对象的值时，装箱的值将被取消装箱，并使用该值创建新的可为 null 的实例。这就是为什么 ReferenceEquals返回 false .

来自docs :

When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable object, not the Nullable<T> object itself. That is, if the HasValue property is true, the contents of the Value property is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable<T> structure initialized to the underlying value.