Why is this cast from interface to class failing?

Question

private Vector2 ResolveCollision(ICollidable moving, ICollidable stationary)
{
    if (moving.Bounds.Intersects(stationary.Bounds))
    {
        if (moving is Player)
        {
            (Player)moving.Color = Color.Red;
        }
    }
    // ...
}

I have a class Player that implements ICollidable. For debugging purposes I'm just trying to pass a bunch of ICollidables to this method and do some special stuff when it's the player. However when I try to do the cast to Player of the ICollidable I get an error telling me that ICollidable doesn't have a Color property.

Am I not able to make a cast this way or am I doing something wrong?

Just a case of parantheses wrong: `((Player)moving).Color = ...` — Davio, Nov 09 '12 at 14:12
you should say ((Player)moving).Color = Color.Red if Color is an attribute of Player — wxyz, Nov 09 '12 at 14:13
My new problem is how I decide which answer to accept. I can never fault this site for being slow... — ssb, Nov 09 '12 at 14:15
though you may have to do this many times as there are only 6 answers and 8 possibilities..aah..sure..just roll a dice — wxyz, Nov 09 '12 at 14:18

Mark Byers · Accepted Answer · 2012-11-09T14:25:45.933

I'd suggest using as instead of is:

Player player = moving as Player;
if (player != null)
{
    player.Color = Color.Red;
}

The advantage is that you only do the type check once.

The specific reason why your code doesn't work (as mentioned in other answers) is because of operator precedence. The . operator is a primary operator which has a higher precedence than the casting operator which is a unary operator. Your code is interpreted as follows:

(Player)(moving.Color) = Color.Red;

Adding the parentheses as suggested by other answers solves this issue, but changing to use as instead of is makes the issue go away completely.

score 9 · Answer 2 · answered Nov 09 '12 at 14:12

9

Your syntax is casting Color to Player, not moving.

((Player)mover).Color = Color.Red;
//^do the cast  ^access the property from the result of the cast

Also, as tends to be a little nicer. If it fails, the result is null:

var player = moving as Player;
if(player != null)
{
    player.Color = Color.Red;
}

answered Nov 09 '12 at 14:12

Rex M

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`is` and then a cast is virtually identical to `as` and then a null check. In neither case will an exception be thrown. – Servy Nov 09 '12 at 14:27

score 3 · Answer 3 · edited Nov 09 '12 at 16:53

3

It's not that it's not working, it's the syntax that is "somewhat" wrong.

Try this:

((Player) moving).Color = Color.Red;

edited Nov 09 '12 at 16:53

LarsTech

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answered Nov 09 '12 at 14:13

Chayemor

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score 2 · Answer 4 · answered Nov 09 '12 at 14:12

2

You should add additional brackets:

((Player)moving).Color = Color.Red;

answered Nov 09 '12 at 14:12

Artem Vyshniakov

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score 2 · Answer 5 · answered Nov 09 '12 at 14:13

2

You need parentheses around the cast and the variable:

((Player)moving).Color = Color.Red;

otherwise you're trying to cast moving.Color to Player.

answered Nov 09 '12 at 14:13

Joey

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score 2 · Answer 6 · answered Nov 09 '12 at 14:13

2

You have forgotten one bracket:

change

 (Player)moving.Color = Color.Red;

to

 ((Player)moving).Color = Color.Red;

You can also use the as operator to cast.

Player p = moving as Player;
if (p != null)
{
    p.Color = Color.Red;
}

answered Nov 09 '12 at 14:13

Tim Schmelter

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Why is this cast from interface to class failing?

6 Answers6