ArrayList

[rn5a]

What's wrong with the following code?

Dim arrColors(5) As ArrayList

arrColors(5) = New ArrayList
arrColors(0).Add("Red")
arrColors(1).Add("Blue")
arrColors(2).Add("Green")
arrColors(3).Add("Yellow")
arrColors(4).Add("Black")

Response.Write(arrColors(0).ToString & "<br>")
Response.Write(arrColors(1).ToString & "<br>")
Response.Write(arrColors(2).ToString & "<br>")
Response.Write(arrColors(3).ToString & "<br>")
Response.Write(arrColors(4).ToString & "<br>")

The above code generates the following error:

Object reference not set to an instance of an object.

pointing to

arrColors(0).Add("Red")

Thanks,

Ron

[Aidy]

arrColors(0).Add("Red")

arrColours(0) is the object at position 0 in the array, however you have not
added anything to the array so this returns null (nothing) and you get your
error.

Do this instead;

arrColors.Add("Red")
arrColors.Add("Blue")
arrColors.Add("Green")
arrColors.Add("Yellow")
arrColors.Add("Black")

[rn5a]

On Oct 16, 3:26 am, "Aidy" <[e-mail]:

arrColors(0).Add("Red")

arrColours(0) is the object at position 0 in the array, however you have not
added anything to the array so this returns null (nothing) and you get your
error.

Do this instead;

arrColors.Add("Red")
arrColors.Add("Blue")
arrColors.Add("Green")
arrColors.Add("Yellow")
arrColors.Add("Black")

Thanks, Aidy, for your suggestion. Why doesn't

arrColors(0).Add("Red"),
arrColors(1).Add("Blue")

etc. add anything to the array? I couldn't exactly understand this.
Could you please explain me?

Also is it possible to ReDim & Preserve with ArrayList?

Thanks once again,

Regards,

Ron

[Aidy]

Thanks, Aidy, for your suggestion. Why doesn't

>
arrColors(0).Add("Red"),
arrColors(1).Add("Blue")

etc. add anything to the array?

Basically an ArrayList is not an array, it is just a collection of objects.
With an array, if I create a 5 length array then I get a structure that can
hold 5 elements of my given type. So I can then do;

arr[0] = "Red"
arr[1] = "Blue"

etc.

When you create an ArrayList, you merely get an object that is an empty
collection, it has no objects in it so you can't do

arr[0] =

as there is nothing at position 0. For there to be something at position 0
you have to add it;

arr.Add("Red") -- this is the first thing added so will have position 0

You're just getting confused between arrays and the ArrayList object,
they're not the same. This syntax;

arrColors(0).Add("Red")

is the same as this

Dim o as Object
o = arrColours(0)
o.Add("Red")

What you're doing is getting the object at position 0 then calling that
object's Add method. As well as the problem above of there being no objects
in the array, if there *was* an object such as a string "Red" then you're
calling the Add method of that object which can give you errors if the
object doesn't support it. You are *not* calling the Add method of the
ArrayList, you are calling the Add method of the object at that position.

Also is it possible to ReDim & Preserve with ArrayList?

Again not really as they are just collections. The ArrayList will grow and
shrink in size as you add/remove elements.