C# Collections - ArrayList

System.Collections.ArrayList

An arraylist is a combination of an array and a collection.
legacy only used for backwards compatibility
Upgrade to Generic.List<T>

This allows you to work with a set of values as if they were an array and a collection at the same time.
An arraylist has the following functionality:

You can address a value by its index
You can quickly sort and reverse the list
You can search sequentially
You can append elements to the end and insert them in a particular position
You can remove any element
Expands automatically as more elements are added (just like a collection)

Imagine that you need to keep track of various objects but you have no idea how many.
It is difficult to use an array for this purpose because you must declare the size of the array at compile time
The ArrayList is a class designed specifically for this situation
This is an aray whose size is dynamically increased as required.

Initailising ArrayLists

The default capacity for a new arraylist is 16 elements.

Dim aliArrayListIntegers As New System.Collections.ArrayList

You should always try and define an initial capacity for your arraylist if possible.
This defines an arraylist with an initial capacity of 200 elements.

Dim aliArrayListIntegers  As New System.Collections.ArrayList(200)

You can modify the capacity at any time by assigning a value to the Capacity property

aliArrayListIntegers.Capacity = aliArrayListInteger.Count

Another way to achieve the same result is to use the TrimToSize property

aliArrayListIntegers.TrimToSize

When the current capacity is exceeded the ArrayList will automatically double its capacity.

Default Elements

It is possible to initialise an arraylist and to poulate it with an initial value

Dim aliArrayListIntegers  As New System.Collections.ArrayList.Repeat(-1,200) 
Dim alsArrayListStrings  As New System.Collections.ArrayList.Repeat("",200)

Properties/Methods

The ArrayList class fully implements the IList interface with all the familiar methods.

Add	Appends an element after the last element in the collection and returns the index where it was inserted.
Capacity	Property contains the number of elements the srray can currently hold.
Clear	Removes all elements from the collection.
Contains	Returns TRUE if an element exists in the collection.
CopyTo	Copies the elements from the collection to a new one-dimensional array, starting at the specified index in the array.
Count	Returns the number of elements in the collection.
GetEnumerator	Method that returns an enumerator to iterate an ArrayList
IndexOf	Returns the index of the element in the collection, or -1 if it cannot be found.
Insert	Inserts an element at the specified index.
IsFixedSize	Returns TRUE if no more elements can be added to the collection.
IsReadOnly	Returns TRUE is the elements cannot be altered.
Item	Gets or sets the elements at the specified 0-based index.
Remove	Removes an element from the collection
RemoveAt	Removes an element at the specified index from the collection
Reverse	Method that reverses the order of the elements
Sort	Method that alphabetically sorts the elements
ToArray	Method that copies all the elements to a new one-dimensional array

Do not return Nothing

Return New System.Collections.ArrayList

Does an Element Exist

If (alArrayList.Contains(sElement) = True) Then 

End If

Removing an Element

The Remove method will only remove the first occurrence of an element.

alAuthors.Remove("")

Also this method does not generate an exception if the element you want to remove cannot be found.
There are two ways you can remove all the instances of an element.

IndexOf

This method is very concise but not very efficient.
You can also use the contain method.

Do While alsArrayListStrings.IndexOf("remove this") >= 0 
   alsArrayListStrings.Remove("remove this") 
Loop

Count

This is far more efficient, this loops until the count property becomes constant.

Dim isavecount As Integer 
Do 
   isavecount = alsArrayListStrings.Count 
   alsArrayListStrings.Remove("remove this") 
Loop While (alsArrayListStrings.Count < isavecount)

Converting to a String Array

Dim arValues() As String 
Dim arArrayList As System.Collections.ArrayList 

arValues = DirectCast(arArrayList.ToArray(GetType(String)),String())