CollectionBase Class

System.Collections

Provides a base class for implementing strongly typed collection classes. This class is abstract.

Syntax

public abstract class CollectionBase : IEnumerable, ICollection, IList

Inheritance Hierarchy

System.Object → System.Collections.CollectionBase

Remarks

The CollectionBase class provides a convenient, abstract base class for implementing custom collection classes. It implements the IList, ICollection, and IEnumerable interfaces. You can derive a custom collection class from CollectionBase to create a strongly typed collection for your application.

CollectionBase internally uses an ArrayList to store the elements of the collection. It provides basic functionality such as adding, removing, and accessing elements. Derived classes typically override methods such as OnInsert, OnRemove, and OnSet to perform custom validation or actions when elements are added, removed, or modified.

Members

Member	Description
CollectionBase()	Initializes a new instance of the `CollectionBase` class.
Count	Gets the number of elements contained in the `CollectionBase` instance.
InnerList	Gets a `ArrayList` wrapper around the collection that provides access to the elements.
Items	Gets the element at the specified index.
OnClear()	Performs custom processing when the collection is cleared. (Protected)
OnInsert(int index, object value)	Performs custom processing before inserting a new element. (Protected)
OnRemove(int index, object value)	Performs custom processing when removing an element. (Protected)
OnSet(int index, object oldValue, object newValue)	Performs custom processing when replacing an element. (Protected)
OnValidate(int index, object value)	Performs custom validation when adding or setting an element. (Protected)

Example

Creating a Custom Collection

The following example demonstrates how to create a custom collection class by deriving from CollectionBase and overriding some of the protected methods for validation.


using System;
using System.Collections;

// Define a custom collection for integers
public class IntegerCollection : CollectionBase
{
    // Override OnInsert to ensure only integers are added
    protected override void OnInsert(int index, object value)
    {
        if (!(value is int))
        {
            throw new ArgumentException("Only integers can be added to this collection.");
        }
        base.OnInsert(index, value);
    }

    // Override OnValidate for additional validation
    protected override void OnValidate(int index, object value)
    {
        if ((int)value < 0)
        {
            throw new ArgumentException("Values must be non-negative.");
        }
        base.OnValidate(index, value);
    }

    // Provide an indexer for easy access to elements
    public int this[int index]
    {
        get { return (int)List[index]; }
        set { List[index] = value; }
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        IntegerCollection myIntCollection = new IntegerCollection();

        try
        {
            myIntCollection.Add(10);
            myIntCollection.Add(25);
            myIntCollection.Add(5);

            Console.WriteLine($"Collection Count: {myIntCollection.Count}");

            Console.WriteLine("Elements in the collection:");
            for (int i = 0; i < myIntCollection.Count; i++)
            {
                Console.WriteLine($"  Index {i}: {myIntCollection[i]}");
            }

            // This will throw an ArgumentException because it's not an integer
            // myIntCollection.Add("hello"); 

            // This will throw an ArgumentException because it's negative
            // myIntCollection.Add(-2);

        }
        catch (ArgumentException ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}


Imports System
Imports System.Collections

' Define a custom collection for integers
Public Class IntegerCollection
    Inherits CollectionBase

    ' Override OnInsert to ensure only integers are added
    Protected Overrides Sub OnInsert(index As Integer, value As Object)
        If Not TypeOf value Is Integer Then
            Throw New ArgumentException("Only integers can be added to this collection.")
        End If
        MyBase.OnInsert(index, value)
    End Sub

    ' Override OnValidate for additional validation
    Protected Overrides Sub OnValidate(index As Integer, value As Object)
        If CType(value, Integer) < 0 Then
            Throw New ArgumentException("Values must be non-negative.")
        End If
        MyBase.OnValidate(index, value)
    End Sub

    ' Provide an indexer for easy access to elements
    Public Property Item(index As Integer) As Integer
        Get
            Return CType(List(index), Integer)
        End Get
        Set(value As Integer)
            List(index) = value
        End Set
    End Property
End Class

Public Class Program
    Public Shared Sub Main(args As String())
        Dim myIntCollection As New IntegerCollection()

        Try
            myIntCollection.Add(10)
            myIntCollection.Add(25)
            myIntCollection.Add(5)

            Console.WriteLine($"Collection Count: {myIntCollection.Count}")

            Console.WriteLine("Elements in the collection:")
            For i As Integer = 0 To myIntCollection.Count - 1
                Console.WriteLine($"  Index {i}: {myIntCollection(i)}")
            Next

            ' This will throw an ArgumentException because it's not an integer
            ' myIntCollection.Add("hello") 

            ' This will throw an ArgumentException because it's negative
            ' myIntCollection.Add(-2)

        Catch ex As ArgumentException
            Console.WriteLine($"Error: {ex.Message}")
        End Try
    End Sub
End Class

Requirements

Namespace: System.Collections
Assembly: System.dll

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