Overview of Collections

Introduction

The .NET Framework supports collections in various namespaces. While the System.Collections namespace provides regular collection classes, the System.Collections.Generic namespace contains the equivalent generic classes. 

One the most routines operations performed on a database consists of reviewing its values. To assist you with this, the .NET Framework provides the IEnumerator and the IEnumerable interfaces that are defined in the System.Collections namespace. Their generic equivalences can be found in the System.Collections.Generic namespace. After implementing these interfaces, you can use the foreach operator to visit each value of the database.

To implement the System.Collections.IEnumerator interface, you must derive a class from it. Then, you must define the Reset(), the MoveNext() methods, and the Current property. Here is an example:

Imports System.ComponentModel

Public Class Enumerator
    Implements IEnumerator

    Private Names() As String
    Private Cur As Integer

    Public Sub New(ByVal lst() As String)
        Names = lst
        Cur = -1
    End Sub

    Public ReadOnly Property Current() Implements IEnumerator.Current
        Get
            Return Names(Cur)
        End Get
    End Property

    Public Sub Reset() Implements IEnumerator.Reset
        Cur = -1
    End Sub

    Public Function Movenext() As Boolean Implements IEnumerator.MoveNext
        Cur = Cur + 1

        If Cur < Names.Length Then
            Return True
        Else
            Return False
        End If
    End Function
End Class

To implement the System.Collections.IEnumerable interface, you must derive a class from it. When implementing the class, you must define an accessory method and the GetEnumerator() method that returns an IEnumerator object. Here is an example: 

Imports System.Collections

Public Class Enumerable
    Implements IEnumerable

    Private Names() As String

    Public Sub Identify(ByVal Values() As String)
        Names = Values
        Dim i As Integer

        For i = 0 To Values.Length - 1
            Names(i) = Values(i)
        Next
    End Sub

    Public Function GetEnumerator() As IEnumerator _
                    Implements IEnumerable.GetEnumerator
        Return New Enumerator(Names)
    End Function
End Class

Once you have implemented the interfaces, you can use For Each. Here is an example:

Public Class Exercise

    Private Sub Exercise_Load(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles MyBase.Load
        Dim strName As String
        Dim FullNames(7) As String

        FullNames(0) = "Gertrude Monay"
        FullNames(1) = "Paul Bertrand Yamaguchi"
        FullNames(2) = "Hermine Ngaleu"
        FullNames(3) = "Francine Mukoko"
        FullNames(4) = "Joseph Walters"
        FullNames(5) = "Patricia Katts"
        FullNames(6) = "Helen Cranston"
        FullNames(7) = "Paul Motto"

        Dim coll As Enumerable = New Enumerable

        coll.Identify(FullNames)

        For Each strName In coll
            lbxNames.Items.Add(strName)
        Next
    End Sub
End Class

While the IEnumerator and the IEnumerable interfaces serve as valuable accessories that allow a collection class to support enumeration, to actually create a collection class, there are other interfaces you can use to implement the functionality you want for your collection.

When you want to use a collection in your application, you may first check what classes are available in the .NET Framework. If you don't find a suitable class, you can create your own that implements one or more interfaces. As it happens, the .NET Framework ships with many of them and your next step is to choose which one you prefer. Some of the most commonly used interfaces are

• System.Collections.IComparer and System.Collections.Generic.IComparer:If you derive a class from this interface, you can define how two objects would be compared for similarity or difference
• System.Collections.IDictionary and System.Collections.Generic.IDictionary: This interface is used to create a collection class where each item is made of a  key=value combination

One of the primary pieces of information you should provide about a the values in a database is the number of values that a list is (currently) holding. When creating a collection class, to prepare it to provide this valuable information, you can (should) implement an interface named ICollection. The ICollection interface is defined in the System.Collections namespace while its equivalent of the same name is defined in the System.Collections.Generic namespace. This means that, if you are creating a class that implements it, you should include this namespace in the file. Here is an example for the System.Collections.ICollection interface:

Public Class BookCollection
    Implements ICollection

End Class

To assist you with keeping track of the number of items in a collection, the ICollection interface is equipped with a property named Count, which you must implement. To do this, you can create a private member variable that will actually keep a count of the number of items. The Count property can then be used to communicate this information to the clients of the class. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer_
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

End Class

The ICollection interface also allows its implementer to copy some of its items to an array. To provide this functionality, the interface is equipped with a method named CopyTo, which you must implement. The syntax of this method is:

Sub CopyTo(array As Array, index As Integer)

This method takes two arguments. The first argument is the array that will receive the items. The second argument is the index of the item from where the copying operation will begin. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private Books() As String
    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer _
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

    Public Sub CopyTo(ByVal ary As Array, ByVal index As Integer) _
                    Implements ICollection.CopyTo
        Dim i As Integer
        Dim bks(Count) As String

        For i = 0 To Count - 1
            bks(i) = Books(i)
            ary = bks
            i = i + 1
        Next
    End Sub
End Class

If you create a collection class, you can provide the ability to enumerate its items. When this is done, some time to time, you will want to identify or to know what item is currently being accessed. In case other collection classes are using the same function at the time you are accessing this information, you should have an object that is responsible for synchronizing the collection. To do this in your ICollection-based class, you must implement a property named SyncRoot. This property must return an Object object. Here is an example:

Public Class BookCollection
    Implements ICollection

    . . . No Change

    Public ReadOnly Property SyncRoot() As Object _
                Implements ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property
End Class

Besides the ability to specify the number of items in a collection, a class that implements the ICollection interface must retrieve a value that indicates whether its item is synchronized. To give this information, you must implement a Boolean property named IsSynchronized. Here is an example:

Public Class BookCollection
    Implements ICollection
	. . . No Change

    Public ReadOnly Property IsSynchronized() As Boolean _
                    Implements ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property
End Class

System.Collections.ICollection (and System.Collections.Generic.ICollection) exdepends the IEnumerable interface. This means that you should be able to use foreach in your ICollection-based class but you must create the functionality yourself, which is done by implementing the GetEnumerator() method. Even if you don't want to support this feature, you still must provide at least a skeleton for this method. Here is an example:

Public Class BookCollection
    Implements ICollection

    Private Books() As String
    Private NumberOfBooks As Integer

    Public Sub New()
        NumberOfBooks = 0
    End Sub

    Public ReadOnly Property Count() As Integer _
                    Implements ICollection.Count
        Get
            Return NumberOfBooks
        End Get
    End Property

    Public Sub CopyTo(ByVal ary As Array, ByVal index As Integer) _
                    Implements ICollection.CopyTo
        Dim i As Integer
        Dim bks(Count) As String

        For i = 0 To Count - 1
            bks(i) = Books(i)
            ary = bks
            i = i + 1
        Next
    End Sub

    Public ReadOnly Property SyncRoot() As Object _
                Implements ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
                    Implements ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public Function GetEnumerator() As IEnumerator _
        Implements IEnumerable.GetEnumerator
        Return Nothing
    End Function
End Class

While it provides the minimum functionality of a collection, the System.Collections.ICollection (and the System.Collections.Generic.ICollection) interface is not equipped to perform the regular operations of a collection class, such as adding, retrieving, or deleting items from a set.

To assist you with creating a collection class as complete as possible, the .NET Framework provides an interface named IList. The IList interface is defined in the System.Collections namespace and its equivalent of the same name is defined in the System.Collections.Generic namespace. The interface is equipped with the methods necessary to add, insert, delete, or retrieve items from a collection. Because the functionalities of these methods may not suit you, to use these features, you must create a class that implements them.

As mentioned above, to create a collection, you can derive it from the IList interface. Here is an example:

Imports System.Collections

Public Class BookCollection
    Implements IList

End Class

This System.Collections.IList interface is declared as follows:

Public Interface IList _
	Implements ICollection, IEnumerable

This System.Collections.Generic.IList interface is declared as follows:

Public Interface IList(Of T) _
	Implements ICollection(Of T), _
	IEnumerable(Of T),  _
	IEnumerable

This means that the IList interface exdends both the ICollection and the IEnumerable interfaces. This also implies that you must implement the members of these parent interfaces. In other words, you must implement the Count property, the SyncRoot property, the IsSynchronized property, and the CopyTo() method of the ICollection interface.

If you use Microsoft Visual Basic 2008 to create the project, as soon as you type the Implements IList line and press Enter, the studio would generate the complete skeleton code for you:

Public Class BookCollection
    Implements IList

    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
				Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
		 Implements System.Collections.ICollection.Count
        Get

        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
		Implements System.Collections.ICollection.IsSynchronized
        Get

        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object  _
		Implements System.Collections.ICollection.SyncRoot
        Get

        End Get
    End Property

    Public Function GetEnumerator() As System.Collections.IEnumerator _
		Implements System.Collections.IEnumerable.GetEnumerator

    End Function

    Public Function Add(ByVal value As Object) As Integer _
		 Implements System.Collections.IList.Add

    End Function

    Public Sub Clear() Implements System.Collections.IList.Clear

    End Sub

    Public Function Contains(ByVal value As Object) As Boolean _
		 Implements System.Collections.IList.Contains

    End Function

    Public Function IndexOf(ByVal value As Object) As Integer _
		 Implements System.Collections.IList.IndexOf

    End Function

    Public Sub Insert(ByVal index As Integer, ByVal value As Object) _
		 Implements System.Collections.IList.Insert

    End Sub

    Public ReadOnly Property IsFixedSize() As Boolean _
		 Implements System.Collections.IList.IsFixedSize
        Get

        End Get
    End Property

    Public ReadOnly Property IsReadOnly() As Boolean _
		 Implements System.Collections.IList.IsReadOnly
        Get

        End Get
    End Property

    Default Public Property Item(ByVal index As Integer) As Object _
		 Implements System.Collections.IList.Item
        Get

        End Get
        Set(ByVal value As Object)

        End Set
    End Property

    Public Sub Remove(ByVal value As Object) _
		Implements System.Collections.IList.Remove

    End Sub

    Public Sub RemoveAt(ByVal index As Integer) _
			Implements System.Collections.IList.RemoveAt

    End Sub
End Class

You can then customize it. From what we learned with ICollection, here are examples of implementing these members for the System.Collections.IList interface:

Public Class BookCollection
    Implements IList

    Private Counter As Integer
    Private Objects(10) As Object

    Public Sub New()
        Counter = 0
    End Sub
      
    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
                      Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
            Implements System.Collections.ICollection.Count
        Get
            Return Counter
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
            Implements System.Collections.ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object _
            Implements System.Collections.ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property
End Class

You must also implement the System.Collections.GetEnumerator() (or the System.Collections.Generic.GetEnumerator()) method of the System.Collections.IEnumerable  (or of the System.Collections.Generic.IEnumerable) interface. If you do not have time to completely implement it, you can simply return Nothing. Here is an example for the System.Collections.IList interface:

Public Function GetEnumerator() As System.Collections.IEnumerator _
                Implements System.Collections.IEnumerable.GetEnumerator
        Return Nothing
End Function

In the next sections, we will see how to add values to a list of a database. As you add or insert values in a list, the Count property grows. If your collection is array-based, when you start it, you specify the number of values that the list will contain. In theory, you cannot add new values beyond that number. In reality, you can increase the size of an array and then add a new item. If your collection is a linked list, you are also not confined to the laws of space (unless your computer runs out of memory).

If you create a list whose number of values must be constant, the user cannot add values beyond the maximum allowed number. Therefore, before adding a value, you can first check whether the collection has a fixed size or not. To give you this information, the IList interface is equipped with a Boolean read-only property named IsFxedSize. This property simply lets the user know whether the collection has a fixed number of items.

Here is an example of implementing this property for the System.Collections.IList interface:

Public ReadOnly Property IsFixedSize() As Boolean _
            Implements System.Collections.IList.IsFixedSize
        Get
            Return False
        End Get
End Property

Most databases are meant to receive new values. If you want, you can create a list that cannot receive new values. To support this, the IList interface is equipped with the Boolean IsReadOnly property. If a list is read-only, it would prevent the clients from adding items to it.

Here is an example of implementing the IsReadOnly property for the System.Collections.IList interface:

Public ReadOnly Property IsReadOnly() As Boolean _
            Implements System.Collections.IList.IsReadOnly
        Get
            Return False
        End Get
End Property

As it should be obvious, the primary operation to perform on a list is to populate it with at least one value. To support this, the System.Collections.IList interface is equipped with a method named Add. Its syntax is:

Function Add(value As Object) As Integer

This method takes one argument as the value to add to the list. If your collection is an array, you can first check that there is still enough room in the list to add a new item. In reality, this is never an issue with the System.Collections.IList interface:

• If there is still room in the collection, the value would be added to the list
• If there is not enough room, the value would simply not be added. There would not be a problem and the program would not crash. In fact, no exception would be thrown if the value was not added because of lack of space. On the other hand, since the compiler would not let you know that there was a problem with "logistic", you may not know whether the value was added or not. Therefore, if you are concerned with knowing whether the value was added, you must provide this functionality yourself 

If the method succeeds with the addition, it returns the position where the value was added in the list. This is usually the last position in the list.

Here is an example:

Public Function Add(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.Add
        ' Check whether there is still room in
        ' the array to add a new item
        If Counter < Objects.Length Then
            ' Since there is room, put the new item to the }
            Objects(Counter) = value
            ' increase the number of items
            Counter = Counter + 1
            ' Return the index of the item that was added
            Return Counter - 1
            ' Since the item could not be added, return a negative index
        Else
            Return -1
        End If
End Function

When you call the System.Collections.IList.Add() method, it adds the new value to the } of the list. Sometimes, you will want the new value to be insert somewhere inside the list. To support this operation, both the System.Collections.IList and the System.Collections.Generic.IList interfaces provide a method named Insert. The syntax of the System.Collections.IList.Insert() method is:

Sub Insert(index As Integer, value As Object)

The syntax of the System.Collections.Generic.IList.Insert() method is:

Sub Insert(index As Integer, item As T)

This method takes two arguments. The second argument is the value that will be inserted into the list. The argument must hold a valid value. Because this method takes an Object object, if your collection is using a different type of value, you may have to cast it to Object. The first argument is the index of the item that will precede the new one.

As mentioned for the System.Collections.IList. Add() method, there are a few things you should know about this operation's success or lack of it:

• If the index argument holds a negative value or a value higher than the allowed number (for example if the list is an array) of the items (depending on how you implement the method), the new value would not be added, the compiler would not throw an exception, and therefore nothing would let you know that the value was not added. If you want to find out whether the value was formally or actually inserted or not, you must create the functionality yourself
• If the value argument is not valid, again depending on how you structure your class, either the value would not be inserted or something else would go wrong. Fortunately, if the value argument is of the type of a class you created yourself, the compiler would produce an error such as letting you know that the argument is holding a value that is not conform to its property or member variable

While using a list, various operations require that you know the object you are currently accessing. To provide this operation, you must create an indexed property. This property should take an index and return the type of object that makes up the list. Here is an example:

Default Public Property Item(ByVal index As Integer) As Object _
                            Implements System.Collections.IList.Item
        Get
            Return Objects(index)
        End Get
        Set(ByVal value As Object)
            Objects(index) = value
        End Set
End Property

After creating this property, you can then access an item using its index and applying the () operator on its instance. Remember that if you want to use For Each, you must appropriately implement the IEnumerable.GetEnumerator() method.

One of the most valuables features of Visual Basic is to use the For Each loop to enumerate the members of a collection. To make this possible, you must implement the IEnumerator interface in your collection class. Following the rules of interface implementation, you must override the members of IEnumerator. 

One of the routine operations you can perform on a list is to find out whether it contains a certain value. To assist you with this operation, the System.Collections.IList interface is equipped with a method named Contains. Its syntax is:

Function Contains(value As Object) As Boolean

This method takes as argument the value to look for. If the value is found in the list, the method returns true. If no value is found in the collection, this method returns false.

Here is an example of implementing this method:

Public Function Contains(ByVal value As Object) As Boolean _
                Implements System.Collections.IList.Contains
    Dim i As Integer

    For i = 0 To Count - 1
        If Objects(i) = value Then
            Return True
            i = i + 1
        End If
    Next

    Return False
End Function

This method calls the Equals() method of the objects that make up the list to find out whether the value argument exists in the collection. If this method produces a wrong result, especially if you are using your own class to represent the item, you may have to override your own Equals() method.

The System.Collections.IList.Contains() method is used to check whether a particular value (already) exists in the collection. If you know that a certain item exists in the collection but you do not know its index inside the list, the IList interface can assist you through a method named IndexOf. Its syntax is:

Function IndexOf(value As Object) As Integer

This method takes as argument the value to look for in the list. If the value is found in the collection, the method returns its index. If there is no value defined like that, the method returns -1. Here is an example of implementing this method:

Public Function IndexOf(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.IndexOf

        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return 1
                i = i + 1
            End If
        Next

        Return -1
End Function

This method calls the Equals() method of the objects that make up the collection to find out whether the value argument exists in the list. If this method produces a wrong result, especially if you are using your own class to represent the value, you may have to override your own Equals() method.

If a value is not necessary in your list, you can delete it. Probably the simplest way to delete a value is to specify its position in the list. To support this operation, both the System.Collections.IList and the System.Collections.Generic.IList interfaces are equipped with a method named RemoveAt. The syntax of the RemoveAt() method is is the same for both interfaces and it is:

Sub RemoveAt(index As Integer)

This method takes as argument the index of the value to be removed. Here is an example:

Public Sub RemoveAt(ByVal index As Integer) _
            Implements System.Collections.IList.RemoveAt
        Dim i As Integer

        If (index >= 0) And (index < Count) Then
            For i = index To Count - 1

                Objects(i) = Objects(i + 1)
                Counter = Counter - 1
            Next
        End If
End Sub

The problem with deleting a value based on its index is that, if you are not sure, you could delete the wrong value. An alternative is to first precisely define the value you want to get rid of, and then hand the value itself to the compiler that would remove it. To support this approach, the System.Collections.IList interface is equipped with a method named Remove() and whose syntax is:

Sub Remove(value As Object)

This method takes as argument the value to be deleted. This means that the compiler will first look for the value in the list. If it finds that value, it removes it. If there is no value like that, nothing happens (the compiler doesn't throw an exception. Here is an example of implementing this method:

Public Sub Remove(ByVal value As Object) _
            Implements System.Collections.IList.Remove
        RemoveAt(IndexOf(value))
End Sub

To remove all value from a list at once, you can implement Clear() method of the System.Collections.IList interface. Its syntax is:

Sub Clear

Here is an example of implementing it:

Public Class BookCollection
    Implements IList

    Private Counter As Integer
    Private Objects(10) As Object

    Public Sub New()
        Counter = 0
    End Sub
      
    Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) _
                            Implements System.Collections.ICollection.CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
            Implements System.Collections.ICollection.Count
        Get
            Return Counter
        End Get
    End Property

    Public ReadOnly Property IsSynchronized() As Boolean _
            Implements System.Collections.ICollection.IsSynchronized
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object _
            Implements System.Collections.ICollection.SyncRoot
        Get
            Return Me
        End Get
    End Property

    Public Function GetEnumerator() As System.Collections.IEnumerator _
                Implements System.Collections.IEnumerable.GetEnumerator
        Return Nothing
    End Function

    Public Function Add(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.Add
        ' Check whether there is still room in
        ' the array to add a new item
        If Counter < Objects.Length Then
            ' Since there is room, put the new item to the }
            Objects(Counter) = value
            ' increase the number of items
            Counter = Counter + 1
            ' Return the index of the item that was added
            Return Counter - 1
            ' Since the item could not be added, return a negative index
        Else
            Return -1
        End If
    End Function

    Public Sub Clear() Implements System.Collections.IList.Clear
        Counter = 0
    End Sub

    Public Function Contains(ByVal value As Object) As Boolean _
                    Implements System.Collections.IList.Contains
        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return True
                i = i + 1
            End If
        Next

        Return False
    End Function

    Public Function IndexOf(ByVal value As Object) As Integer _
                    Implements System.Collections.IList.IndexOf

        Dim i As Integer

        For i = 0 To Count - 1
            If Objects(i) = value Then
                Return 1
                i = i + 1
            End If
        Next

        Return -1
    End Function

    Public Sub Insert(ByVal index As Integer, ByVal value As Object) _
                            Implements System.Collections.IList.Insert

    End Sub

    Public ReadOnly Property IsFixedSize() As Boolean _
            Implements System.Collections.IList.IsFixedSize
        Get
            Return False
        End Get
    End Property

    Public ReadOnly Property IsReadOnly() As Boolean _
            Implements System.Collections.IList.IsReadOnly
        Get
            Return False
        End Get
    End Property

    Default Public Property Item(ByVal index As Integer) As Object _
                            Implements System.Collections.IList.Item
        Get
            Return Objects(index)
        End Get
        Set(ByVal value As Object)
            Objects(index) = value
        End Set
    End Property

    Public Sub Remove(ByVal value As Object) _
            Implements System.Collections.IList.Remove
        RemoveAt(IndexOf(value))
    End Sub

    Public Sub RemoveAt(ByVal index As Integer) _
            Implements System.Collections.IList.RemoveAt
        Dim i As Integer

        If (index >= 0) And (index < Count) Then
            For i = index To Count - 1

                Objects(i) = Objects(i + 1)
                Counter = Counter - 1
            Next
        End If
    End Sub
End Class