Possible to Create an ArrayList of Objects in C#/.NET?

Question

I'm trying to create an ArrayList of objects in C#. What I've tried so far is:

class POObject
    {
        public List<string> staticCustInfo;
        public List<List<string>> itemCollection;
        public int testInt;
    }

    POObject myObject = new POObject();
    List<POObject> BatchList = new List<POObject>();

This is fine, except when I try to add an object to this list using:

BatchList.Add(item);

It gives me errors saying it can't find a corresponding method to add to the list. Any ideas? Thanks for the help.

Just for clarity, you aren't creating an ArrayList. An ArrayList is a different class from List<>. — GendoIkari
– GendoIkari, Commented Nov 11, 2010 at 17:15
List in .NET is implemented with an array list implementation. — StriplingWarrior
– StriplingWarrior, Commented Nov 11, 2010 at 17:19
Fyi, no repro. New c# solution, paste in the class definition for POObject in the Program class above the Main function, paste in these 3 lines into Main: POObject myObject = new POObject(); List<POObject> BatchList = new List<POObject>(); BatchList.Add(myObject); // no compile or runtime errors — xcud
– xcud, Commented Nov 11, 2010 at 17:19

StriplingWarrior · Accepted Answer · 2010-11-11 17:22:34Z

1

What are you trying to store in your list? You declare a BatchList as containing POObjects, but then you're trying to add item, which is of what type? If you're adding things to this list that are not POObjects, you can declare it as either a non-generic list:

List BatchList = new List();

or as a generic list of objects:

List<object> BatchList = new List<object>();

But I suspect the real problem may be that you're trying to add the wrong type of item to your collection. Can you show us how you get your item variable?

edited Nov 11, 2010 at 17:22

answered Nov 11, 2010 at 17:14

StriplingWarrior

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3 Comments

Dan J Over a year ago

Why would this be necessary? Shouldn't a generic List<T> be able to contain his POObject?

dexter Over a year ago

@djacobson yes List<T> can store any objects, but you have to first instantiate it with a concrete type parameter - T

Dan J Over a year ago

@Max Yes, which the OP is already doing. Now that @StriplingWarrior has extended his answer from just containing that List<object> statement, I agree with it - the problem may very well be that item is an arbitrary type...

Matt Greer · Accepted Answer · 2010-11-11 17:18:53Z

0

If you declare that BatchList is of type List<POObject>, then that means the List can only contain objects of type POObject. That's what the type parameter is for. You cannot add strings, ints, or any other arbitrary objects to this list, only POObjects (and descendents of POObject).

If you take a look at the intellisense, you will see that Add's signature is this:

void Add(POObject item)

If you really want a list that contains arbitrary items, then use an ArrayList, which is not a generic type and can accept any type of object. However since C# is such a strongly typed language, wanting arbitrary lists like this is usually a code smell.

answered Nov 11, 2010 at 17:18

Matt Greer

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