How to cast object from an array

Question

A simple question...

I have an abstract class Cell and two classes BorderCell and BoardCell, which inherit from the Cell class. Then I have a Cells array with the type of Cell[], which contains objects of the type BorderCell and BoardCell.

abstract class Cell 
{
}
class BorderCell : Cell 
{
    public void Method1(){};
}
class BoardCell: Cell
{
    public void Method2(){};
}

...

Cell[] Cells = new Cell[x];
for (int i = 0; i < x; i++){
    Cells[i] = new BorderCell();
    // or
    Cells[i] = new BoardCell();
}

Now I want to cast a cell to BorderCell and run its Method1, like this:

(Border)Cells[i].Method1();

But this doesn't work, I have to use:

BorderCell borderCell = (BorderCell)Cells[i];
borderCell.Method1();

Is this the only (and the right way) to do this)?

I would review your design here -- having to cast back up to derived classes is a bad design smell -- if you're saving them in the same collection as a base type, I would expect them to be truly polymorphic (Liskov Principle). — Dr Herbie
– Dr Herbie, Commented Jun 30, 2010 at 9:32
Dr Herbie: I am open to suggestions, view this thread: stackoverflow.com/questions/3140281/… — sventevit
– sventevit, Commented Jun 30, 2010 at 9:47

Jon Skeet · Accepted Answer · 2010-06-30 09:24:37Z

7

No, you just need brackets to make it clear what you want the cast to apply to:

((Border)Cells[i]).Method1();

Basically the "." binds tighter than the cast, so your original code:

(Border)Cells[i].Method1();

is equivalent to:

(Border)  (Cells[i].Method1());

answered Jun 30, 2010 at 9:24

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Noldorin Over a year ago

Jon is quick on the trigger today (as usual)!

Adam Houldsworth · Accepted Answer · 2010-06-30 09:24:55Z

4

Try:

((BorderCell)Cells[i]).Method1();

The brackets provide type boundaries if you use them with casting. Your first attempt didn't wrap the Cell[i].

answered Jun 30, 2010 at 9:24

Adam Houldsworth

64.8k11 gold badges161 silver badges190 bronze badges

Comments

Christian · Accepted Answer · 2010-06-30 09:27:03Z

3

When you write

(BorderCell)Cells[i].method1();

the cast is applied to the expression Cells[i].method1();, which obviously won't work since Cells[i] still returns a Cell.

If you want to spare an additional variable, then write:

((BorderCell)Cells[i]).method1();

answered Jun 30, 2010 at 9:27

Christian

4,34225 silver badges25 bronze badges

Comments

Arseny · Accepted Answer · 2010-06-30 09:33:38Z

2

Since you put cells of 2 types (BorderCell and BoardCell) in the array. I would recommend check the type first before casting.

if (Cells[i] is BorderCell)
{
// cast to BorderCell
}
else
{
// cast to BoardCell
}

answered Jun 30, 2010 at 9:33

Arseny

7,3514 gold badges39 silver badges52 bronze badges

2 Comments

Jon Skeet Over a year ago

That depends. If the code knows where there should be BorderCells, so is confident that if it's the wrong type, that indicates a bug, then a cast is the right way to do it.

sventevit Over a year ago

True, business logic insists, that if a cell is in a specific loaction, then it should have the right type. See: stackoverflow.com/questions/3140281/…

Collectives™ on Stack Overflow

How to cast object from an array

4 Answers 4

1 Comment

Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related