Object.ReferenceEquals永远不会命中

Question

Can anyone tell me why the following condition does not hit?

List<DateTime> timestamps = new List<DateTime>();
timestamps.Add(DateTime.Parse("8/5/2011 4:34:43 AM"));
timestamps.Add(DateTime.Parse("8/5/2011 4:35:43 AM"));
foreach(DateTime x in timestamps)
{
    if (Object.ReferenceEquals(x, timestamps.First()))
    {
        // Never hit
        Console.WriteLine("hello");
    }
}

Answer 1

Because DateTime is a value type, immutable and so the references will not be equal even though the values are.

Are you meaning to do something like this? Value comparison:

if (DateTime.Compare(x, timestamps.First()) == 0)
{
    // Never hit
    Console.WriteLine("hello");
}

Answer 2

Value types are passed and compared by value . That's why they're called "value types".

Reference types are passed and compared by reference . That's why they're called "reference types".

DateTime is a value type .

Therefore you are attempting to compare two values by reference . That's not going to work. It will always be false.

Can you explain why you'd expect something different?

Answer 3

I think that a number of the other answers miss something.

In the case of object.ReferenceEquals(object,object) any value-types are "boxed" to objects and it is these (new) objects which are passed. Consider the following:

DateTime d1 = DateTime.MinValue;
DateTime d2 = d1;
object ob1 = (object)d1; // boxed!
object ob2 = ob1;

// false - the values in d1 and d2 are BOXED to (new) different objects
object.ReferenceEquals(d1, d2);
// false - same as above, although I am not sure sure if a
//         VM implementation could re-use a BOXED object
object.ReferenceEquals(d1, d1);
// true - naturally - BOXED only once at "boxed!" (same object!)
object.ReferenceEquals(ob1, ob2);

Happy coding.

---

Related: Marc's answer in Value Type Vs Reference Type - Object Class C# where he talks about a boxing conversion .

Answer 4

They have the same value, but they aren't the same reference.

Take a look at the MSDN entry for Object.ReferenceEquals

Furthermore, even if you had been comparing the same object, you would still not hit it by comparing the references, because DateTime is a structure, that is, it is value type. Value Types , by definition, copy the contained value, not the reference to the object (like Reference Types do).

Answer 5

对象首先在ReferenceEquals中复制，因为它们是值类型，因此引用永远不会相等。