Trying to call method of object indexed in ArrayList in C#

Question

I'm new to C# and I'm writing a program where I have an ArrayList (unitArray) of Unit objects and am trying to call a non-static method on the object referenced in the ArrayList. I try to access the specific object and call it's method but it doesn't work. I'd be grateful for help resolving this issue.

Unit.unitArray[selectedUnit].DisplayUnitAttributes()

I get the following exception:

'object' does not contain a definition for 'DisplayUnitAttributes' and no extension method 'DisplayUnitAttributes' accepting a first argument of type 'object' could be found (are you missing a using directive or an assembly reference?)

Though you've got different answers already, I guess you should concern tha answer by cdiggins, if all the element of your ArrayList are of one and the same type, as there is no sense in a more common ArrayList then. — horgh
– horgh, Commented Sep 17, 2012 at 3:05

Valamas · Accepted Answer · 2012-09-17 02:41:21Z

5

you need to cast the object as its type. In place of MyClass below, substitute your actual class type in.

(Unit.unitArray[selectedUnit] as MyClass).DisplayUnitAttributes()

answered Sep 17, 2012 at 2:41

Valamas

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3 Comments

robertito Over a year ago

Word. My Man! Thanks a bunch. Is this casting something special with ArrayLists? I suspect this is because these could contain objects of different types?

Scott Earle Over a year ago

Or you could cast it as ((MyClass)Unit.unitArray[selectedUnit]).DisplayUnitAttributes(), which would throw an invalid cast exception if the object is the wrong type, instead of the generic exception thrown by null object references.

Valamas Over a year ago

@user1676520: correct, the arraylist stores objects. so when you retrieve you have to cast it from an object.

cdiggins · Accepted Answer · 2012-09-17 02:49:59Z

3

The type of elements extracted from an ArrayList are System.Object which is the base class of all objects in C#.

You have to cast element to a derived type to access the methods or better yet use a System.Generic.List<T> where T is the type of the elements in the list.

answered Sep 17, 2012 at 2:49

cdiggins

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Comments

horgh · Accepted Answer · 2012-09-17 02:58:39Z

1

You could use Enumerable.OfType Method to get the necessary subarray. Something like this:

foreach (YourClass obj in Unit.unitArray.OfType<YourClass>())
    obj.DisplayUnitAttributes();

answered Sep 17, 2012 at 2:58

horgh

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2 Comments

Valamas Over a year ago

I like this. Never used it before.

horgh Over a year ago

@Valamas In love with it too. It's very useful to filter "common-typed" collections with this. On the contrary, I almost never use Enumerable.Cast<T> method.

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