Constructors versus Initializors in C#

Question

Possible Duplicate:
What's the difference between an object initializer and a constructor?

In c# you can construct an object like:

public class MyObject{

     int val1;
     int val2;

     public MyObject(int val1, val2){
           this.val1 = val1;
           this.val2 = val2;
     }
}

With:

MyObject ob = new MyObject(1,2);

or with:

MyObject ob = new MyObject(){ val1 = 1, val2 = 2 };

What is the diference between that kind of constructors?

I asked this very same question a while ago. http://stackoverflow.com/questions/740658/whats-the-difference-between-an-object-initializer-and-a-constructor Check there for some more in depth info in addition to the answers here. — Pete, Apr 19 '11 at 14:22
It's amazing that the mistake of "new Object()" instead of "new MyObject()" has made it into every single answer. — juharr, Apr 19 '11 at 14:27

score 18 · Accepted Answer · edited May 18 '12 at 13:58

18

MyObject ob = new MyObject(){ val1 = 1, val2 = 2 };

is just syntactic sugar (i.e. shorthand) for

MyObject ob = new MyObject();
ob.val1 = 1;
ob.val2 = 2;

One difference between the two is that you can set readonly fields from the constructor, but not by using the shorthand.

A second difference is that a constructor with parameters forces the client to provide those values. See Constructor-injection vs. Setter injection for a good bit of background reading.

edited May 18 '12 at 13:58

Sebastian

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answered Apr 19 '11 at 14:19

RB.

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note that the () can be left out here. So MyObject ob = new Object(){ val1 = 1, val2 = 2 }; can be MyObject ob = new Object{ val1 = 1, val2 = 2 }; – Rob Apr 19 '11 at 14:20

score 6 · Answer 2 · answered Apr 19 '11 at 14:20

6

The difference is probably that the second won't compile.

You are missing a default constructor, which it calls in your second example.

answered Apr 19 '11 at 14:20

Arcturus

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+1 good call! The default ctor needs to be defined – joe_coolish Apr 19 '11 at 14:22
There was a mistake new MyObject beside new Object – Daniel Gomez Rico Apr 19 '11 at 15:18

kemiller2002 · Answer 3 · 2011-04-19T14:35:04.673

MyObject ob = new Object(1,2);

This is constructing the object, and doing something with the values in the constructor. It maybe setting the values, it may not. It depends on the logic in the constructor.

MyObject ob = new Object(){ val1 = 1, val2 = 2 };

This is constructing the object using the default constructor, and setting initial values for properties, after the object has been constructed. In this scenario, setting the values has nothing to do with the constructor besides the fact that it is in the same statement. It's the same as:

MyObject ob = new Object();
ob.val1 = 1;
...

What is neat about this syntax is that you can do:

 Object ob = new Object(){ val1 = 1, val2 = 2 };

instead of having to do:

Object ob = new Object(1,2);
((MyObject)ob).val1 = 1;  //note having to cast.
...

because the initialization of the variables happens without touching the variable assignment.

score 1 · Answer 4 · answered Apr 19 '11 at 14:20

1

The second form

  MyObject ob = new Object(){ val1 = 1, val2 = 2 };

is the syntax for

  MyObject ob = new Object();
  ob.val1 = 1;
  ob.val2 = 2;

the key point is that the setting of the values is not part of the constructor.

answered Apr 19 '11 at 14:20

Scott Weinstein

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joe_coolish · Answer 5 · 2011-04-19T14:44:42.087

1

The first is a parameter ctor

MyObject ob = new MyObject (1,2);

The second is the default (zero parameter) ctor, but once the ctor finishes, val1 and val2 are initialized with the values provided

MyObject ob = new MyObject (){ val1 = 1, val2 = 2 };

Also, you can call the second one w/o the braces. E.g.

MyObject ob = new MyObject { val1 = 1, val2 = 2 };

edited Apr 19 '11 at 14:44

answered Apr 19 '11 at 14:20

joe_coolish

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score 1 · Answer 6 · answered Apr 19 '11 at 14:23

Both aren't the same thing.

This is invoking class constructor:

MyObject ob = new Object(1,2);

And this is an inline object initializer:

MyObject ob = new Object(){ val1 = 1, val2 = 2 };

First ensures creating an instance of MyObject requires two integer input parameters during construction time, while the other one, just initializes some fields or properties after construction time. Last one keeps freedom of not necessarily initialize such properties and/or fields.

Constructors versus Initializors in C#

6 Answers6