C# DateTime always create new object?

Question

why in C# my two variables points to different DateTime objects?

DateTime a1 = DateTime.Now;
DateTime a2 = a1;

a1 = a1 + TimeSpan.FromMinutes(15);
a2 = a2 - TimeSpan.FromMinutes(16);

I realized that a2 actually points to a new object which is different from a1.

But In other case. Say i have a class Person, and age = 1;

Person a1 = new Person();
a2 = a1;
a2 = Person.Age = 2;

In the Person Case, a1 and a2 are pointing to same object. I am really confused here, anyone can explain?

Answer 1

DateTime is a value type - a structure.

With value types, when you do something like:

DateTime a2 = a1;

a2 gets a copy of the values of a1. It is not the same reference to the same memory location, but a complete, separate copy.

On the other hand, Person is a reference type - a class.

When you do:

Person p2 = p1;

With a reference type, the reference that p2 points to is the same one that p1 points to. So changes in one are changes to both.

See Value Types and Reference Types on MSDN.

Answer 2

As others have already pointed out DateTime is a struct, not a class and therefore a value type. You can visualize this in the Visual Studio editor, if you change the text color used to display structs. Open the dialog in menu Tools > Options and navigate to Environment > Fonts and Colors

It is helpful to change the color of delegates, enums, interfaces and structs (Value types).

In Visual Studio 2019, you can also change the look of User Members like constants or parameters.