C# constructors overloading

Question

How I can use constructors in C# like this:

public Point2D(double x, double y)
{
    // ... Contracts ...

    X = x;
    Y = y;
}

public Point2D(Point2D point)
{
    if (point == null)
        ArgumentNullException("point");
    Contract.EndContractsBlock();

    this(point.X, point.Y);
}

I need it to not copy code from another constructor...

score 219 · Answer 1 · answered Apr 05 '11 at 17:13

219

public Point2D(Point2D point) : this(point.X, point.Y) { }

answered Apr 05 '11 at 17:13

Mark Cidade

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4

point cannot be null because it is not defined as a nullable type (Point2D? point) – RvdK Apr 05 '11 at 17:16
Describe what you mean by using Contracts. Mark's answer is right. – SQLMason Apr 05 '11 at 17:17
2

@PowerRoy, you are assuming `Point2D` is a value type. If it's a class it may be null. – GeReV Apr 05 '11 at 17:18
@PowerRoy: point can be null because... we don't know how it is defined. – R. Martinho Fernandes Apr 05 '11 at 17:19
No, Point2D may be null. But second constructor copy the point, so parameter point can not be null. – Aleksandr Vishnyakov Apr 05 '11 at 17:20
You should make your Point2D a struct. – Niels Abildgaard Nov 22 '13 at 01:01
2

Thanks.. For other people: If you pass just ": this" with no arguments - for example, the both - empty constructor code plus the overloading constructor code will run. Useful in case that the overloading constructor has the same code as the empty + new code. – Mosh Feu Sep 29 '14 at 14:34

João Angelo · Accepted Answer · 2011-04-05T17:21:19.530

71

You can factor out your common logic to a private method, for example called Initialize that gets called from both constructors.

Due to the fact that you want to perform argument validation you cannot resort to constructor chaining.

Example:

public Point2D(double x, double y)
{
    // Contracts

    Initialize(x, y);
}

public Point2D(Point2D point)
{
    if (point == null)
        throw new ArgumentNullException("point");

    // Contracts

    Initialize(point.X, point.Y);
}

private void Initialize(double x, double y)
{
    X = x;
    Y = y;
}

edited Apr 05 '11 at 17:21

answered Apr 05 '11 at 17:16

João Angelo

56,552
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20

As an avid fan of immutability, I'd like to note that a drawback of this method is that you cannot initialize readonly fields :( – R. Martinho Fernandes Apr 05 '11 at 17:28
I was thinking of this exact same solution, and now you confirmed it's a good idea. Thanks :) – superiggy Mar 23 '13 at 01:12
9

Try Mark Cidade's answer first. Use this one as a fallback. – gidmanma Sep 04 '13 at 15:35

score 6 · Answer 3 · answered Apr 05 '11 at 17:20

Maybe your class isn't quite complete. Personally, I use a private init() function with all of my overloaded constructors.

class Point2D {

  double X, Y;

  public Point2D(double x, double y) {
    init(x, y);
  }

  public Point2D(Point2D point) {
    if (point == null)
      throw new ArgumentNullException("point");
    init(point.X, point.Y);
  }

  void init(double x, double y) {
    // ... Contracts ...
    X = x;
    Y = y;
  }
}

C# constructors overloading

3 Answers3

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