Create object with new and without new

Question

I started to learn C# and I found out that there are two different ways to create the object. First is this:

 Box Box1 = new Box();   // Declare Box1 of type Box
 Box Box2 = new Box();   // Declare Box2 of type Box

Other is this:

 Box Box1 ;   // Declare Box1 of type Box
 Box Box2 ;   // Declare Box2 of type Box

Both methods work, what is the difference? Is there something similar with C++ pointer?

Box* Box1 = new Box();   // Declare Box1 of type Box
Box* Box2 = new Box();   // Declare Box2 of type Box

Your second snippet declares two variables, but doesn't create any objects. — Jon Skeet, Apr 26 '19 at 08:32
Box is a class, then the Box1 and Box2 are the objects in the second method right? what is mean by the instantiate an object? — Ah Tan, Apr 26 '19 at 08:39
`Box Box1; Console.WriteLine(Box1);` will give you "use of unassigned local variable", demonstrating that the statement declares but does not instantiate. (When used as a field, `Box Box1`would be considered initialized, but with `null`, not an object.) — Jeroen Mostert, Apr 26 '19 at 08:40
Is `Box` a `class` or a `struct` ? Makes all the difference in C#. — Henk Holterman, Apr 26 '19 at 08:52
@HenkHolterman: you can't use `struct` variables without initializing them either (with the very limited exception of a `struct` that has no members). — Jeroen Mostert, Apr 26 '19 at 08:58
@JeroenMostert - the OP isn't using them yet. I can't tell from the code. — Henk Holterman, Apr 26 '19 at 10:10

kara · Accepted Answer · 2019-04-26T09:00:56.067

Your second example declares a variable, but it will be empty and can't be accessed:

Box b;
int id = b.Id; // Compiler will tell you that you're trying to use a unassigned local variable

We can fool the compiler by initializing with null:

Box b = null; // initialize variable with null
try
{
    int id = b.Id; // Compiler won't notice that this is empty. An exception will be trown
}
catch (NullReferenceException ex)
{
    Console.WriteLine(ex);
}

We see now, that we have to initialize the variable in order to access it:

Box b; // declare an empty variable
b = new Box(); // initialize the variable

int id = b.Id; // now we're allowed to use it.

Short version of declare and initialize is your 1st example:

Box b = new Box();

Here the example-class i used for the examples:

public class Box
{
    public int Id { get; set; }
}

Perhaps you did notice that Id in our Box is not being initialized. This is not needed (but most times you should do) because it is a value-type (struct) and not a refenrence-type (class).

If you want to read more, have a look at this Question: What's the difference between struct and class in .NET?

You assume Box is a class. What if it is a Value Type instead? — Henk Holterman, Apr 26 '19 at 08:53
Correct. Added an explaination for not initializing Id because it is a `struct`. I'll have a look for the Link to the MS Docs.. — kara, Apr 26 '19 at 08:55
"Compiler will tell you that this is illegal, because "b" is "null"." - no, the compiler will tell you that this is illegal because the variable isn't *definitely assigned*. That's very different from it being defined with a null value. — Jon Skeet, Apr 26 '19 at 08:59

Create object with new and without new

1 Answers1