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Introduction to Built-In Collection Classes

   

Introduction to List Classes

 

Overview

To support the creation of any kinds of list, the Microsoft .NET Framework provides the ArrayList and the generic List classes.

The ArrayList class is defined in the System.Collections namespace while the generic List class is part of the System.Collections.Generic namespace. Therefore, in order to use one of these classes in your application, you can first include its namespace in the file that would perform the list-related operations.

The ArrayList class implements the IList, the ICollection, and the IEnumerable interfaces. The List class implements the generic IList<>, the generic ICollection<>, the generic IEnumerable<>, the IList, the ICollection, and the IEnumerable interfaces.

The ArrayList class is founded as follows:

public class ArrayList : IList,
			 ICollection,
			 IEnumerable, 
			 ICloneable

The generic List class is founded as follows:

public class List<T> : IList<T>,
		       ICollection<T>, 
		       IEnumerable<T>,
		       IList,
		       ICollection,
		       IEnumerable

You can use either the ArrayList or the generic List class to create and manage values for a list. Here is an example of declaring an ArrayList variable:

using System;
using System.Collections;
using System.Windows.Forms;

public class Exercise : Form
{
    ArrayList lstNames;

    public Exercise()
    {
        InitializeComponent();
    }

    void InitializeComponent()
    {
        Load += new EventHandler(StartForm);
    }

    void StartForm(object sender, EventArgs e)
    {
        lstNames = new ArrayList();
    }
}

public class Program
{
    static int Main()
    {
        System.Windows.Forms.Application.Run(new Exercise());
        return 0;
    }
}

Besides the ability to create a list, both the ArrayList and the List classes have the built-in mechanism for serialization.

The default constructor allows you to create an empty list before adding values to it. If you already have an ICollection-based list, that is, a list created from a class that implements the ICollection interface, you can initialize your ArrayList list with it. To support this, the ArrayList class is equipped with the following constructor:

public ArrayList(ICollection c);

Here is an example:

void StartForm(object sender, EventArgs e)
{
    ComboBox cbx = new ComboBox();
    cbx.Items.Add("Paul Bertrand Yamaguchi");
    cbx.Items.Add("Helene Mukoko");
    cbx.Items.Add("John Hancock");
    cbx.Items.Add("Gertrude Monay");

    lstNames = new ArrayList(cbx.Items);
}

The Capacity of a List

After declaring an ArrayList or a List variable, it is empty. As objects are added to it, the list grows. The list can grow tremendously as you wish. The number of items of the list is managed through the memory it occupies and this memory grows as needed. The number of items that the memory allocated is currently using is represented by the Capacity property. Here is an example of accessing the ArrayList.Capacity property:

public class Exercise : Form
{
    ArrayList lstNames;

    public Exercise()
    {
        InitializeComponent();
    }

    void InitializeComponent()
    {
        Load += new EventHandler(StartForm);
    }

    void StartForm(object sender, EventArgs e)
    {
        lstNames = new ArrayList();
        Text = "List Capacity: " + lstNames.Capacity.ToString();
    }
}

This would produce:

ArrayList Capacity

The capacity of a list will usually be the least of your concerns. If for some reason, you want to intervene and control the number of items that your list can contain, you can manipulate the Capacity property. For example, you can assign it a constant to set the maximum value that the list can contain. Instead of specifying the capacity after the list has been created, when declaring the list variable, you can specify its maximum capacity. To support this, both the ArrayList and the List classes are equipped with an additional constructor as follows:

public ArrayList(int capacity);
public List(int capacity);

Once again, you will hardly have any reason to use the Capacity property: the compiler knows what to do with it.

A Read-Only List

One of the reason for creating a list is to be able to add values to it, edit its values, retrieve a value, or delete values from it. These are the default operations. You can still limit these operations as you judge them unnecessary. For example, you may create a list and then initialize it with the values that you want the list to only have. If you do not want to have the user adding values to it, you can create the list as read-only. To do this, you can call the ArrayList.ReadOnly() method. It is overloaded with two versions as follows:

public static ArrayList ReadOnly(ArrayList list)
public static IList ReadOnly(IList list)

Some operations cannot be performed on a read-only list. To perform such operations, you can first find out whether an ArrayList list is read-only. This is done by checking its IsReadOnly property.

Item Addition

The primary operation performed on a list is to create one or more values. To do this, you have various alternatives. Both the ArrayList and the List classes are equipped with an Add() method. The syntax of the System.Collections.ArrayList.Add() method is:

public virtual int Add(object value);

The syntax of the System.Collections.Generic.List.Add() method is:

public void Add(T value);

The argument of the method is the value to add to the list. If the method succeeds with the addition, it returns the position where the value was added in the list. Here are example for an ArrayList variable:

void StartForm(object sender, EventArgs e)
{
    Text = "Employees Records";

    lstNames = new ArrayList();

    lstNames.Add("Christine Kingston");
    lstNames.Add("Hermine Paulson");
    lstNames.Add("William Harrison");
    lstNames.Add("Ursula Morales");
    lstNames.Add("Evan Lancos");
}

If the method fails to add the value and if you are using the ArrayList class, the compiler would throw an error. One of the errors that could result from the ArrayList's failure of this operation would be based on the fact that either a new value cannot be added to the list because the list is read-only, or the list was already full prior to adding the new value. Normally, a list can be full only if you had specified the maximum number of items it can contain using the ArrayList.Capacity property. As mentioned above, the list can be made read-only by passing its variable to the ArrayList.ReadOnly() method.

Instead of adding one values at a time, you can first create a list of values and add that whole list at once. To support this operation, both the ArrayList and the List classes are equipped with a method named AddRange.

The syntax of the ArrayList.AddRange() method is:

public virtual void AddRange(ICollection collection);

The syntax of the List.AddRange() method is:

public void AddRange(IEnumerable<T> collection);

The ArrayList.AddRange() method takes as argument a list created from a class that implements the ICollection interface. Here is an example:

void StartForm(object sender, EventArgs e)
{
    Text = "Employees";

    ComboBox cbx = new ComboBox();
    cbx.Items.Add("Paul Bertrand Yamaguchi");
    cbx.Items.Add("Helene Mukoko");
    cbx.Items.Add("John Hancock");
    cbx.Items.Add("Gertrude Monay");

    lstNames = new ArrayList();
    lstNames.AddRange(cbx.Items);
}

The List.AddRange() method takes as argument a list created from a class that implements the generic IEnumerable interface.

The Number of Items in the List

When using a list, at any time, you should be able to know the number of items that the list contains. This information is provided by the ArrayList.Count or the List.Count property.

The Capacity and the Count properties have this in common: the value of each increases as the list grows and the same value decreases if the list shrinks. It is important to know that there are various differences between the capacity of a list and the number of items it contains. Capacity is a read/write property. This means that you can assign a value to the capacity to fix the number of items that the list can contain. You can also retrieve the value of the Capacity. The Count property is read-only because it is used by the compiler to count the current number of values of the list and this counting is performed without your intervention.

Item Retrieval

Once a list is ready, you can perform different types of operations on it. Besides adding items, one of the most regular operations performed on a list consists of locating and retrieving its values. You have various options.

To give you access to each member of their list, both the ArrayList and the List classes are equipped with the default Item property. The Item property is an indexer. The first value of the list has an index of 0. The second has an index of 1, and so on. 

To retrieve a single value based on its position, you can apply the square brackets of arrays to the variable. Here is an example:

using System;
using System.Drawing;
using System.Collections;
using System.Windows.Forms;

public class Exercise : Form
{
    ArrayList lstNames;
    ListBox lbxNames;

    public Exercise()
    {
        InitializeComponent();
    }

    void InitializeComponent()
    {
        lbxNames = new ListBox();
        lbxNames.Location = new Point(12, 12);
        Controls.Add(lbxNames);

        Load += new EventHandler(StartForm);
    }

    void StartForm(object sender, EventArgs e)
    {
        Text = "Employees";

        lstNames = new ArrayList();

        lstNames.Add("Christine Kingston");
        lstNames.Add("Hermine Paulson");
        lstNames.Add("William Harrison");
        lstNames.Add("Ursula Morales");
        lstNames.Add("Evan Lancos");

        for (int i = 0; i < 5; i++)
            lbxNames.Items.Add(lstNames[i]);
    }
}

public class Program
{
    static int Main()
    {
        System.Windows.Forms.Application.Run(new Exercise());
        return 0;
    }
}

ArrayList

Another issue to keep in mind is that the ArrayList[] indexer returns an Object value. Therefore, you may have to cast this value to your type of value to get it right.

Besides using the index to access a value from the list, the ArrayList and the List classes implement the IEnumerable.GetEnumerator() method. For this reason, you can use the foreach loop to access each member of the collection. Here is an example:

void StartForm(object sender, EventArgs e)
{
    Text = "Employees";

    lstNames = new ArrayList();

    lstNames.Add("Christine Kingston");
    lstNames.Add("Hermine Paulson");
    lstNames.Add("William Harrison");
    lstNames.Add("Ursula Morales");
    lstNames.Add("Evan Lancos");

    foreach (string str in lstNames)
        lbxNames.Items.Add(str);
}

You can use the Item property to change a value in the list. Because the Item property is used to access an existing value from the list, the value must have been created. If you try setting the value of a non existing item, the compiler would throw an ArgumentOutOfRangeException Exception. Here is an example:

void StartForm(object sender, EventArgs e)
{
    Text = "Employees";

    lstNames = new ArrayList();

    lstNames[0] = "Paul Bertrand Yamaguchi";
}

Notice that at the time the 0 item is accessed, it has not previous been created. This would produce:

Error

A review of the Details section shows:

System.ArgumentOutOfRangeException: Index was out of range. 
Must be non-negative and less than the size of the collection.
Parameter name: index
at System.Collections.ArrayList.set_Item(Int32 index, Object value)
at Exercise.StartForm(Object sender, EventArgs e) 
in E:\Programs\VCSharp\Exercise1\Exercise1\Exercise.cs:line 31

This means that you can use the Item property only to change the value of a previous created item.

Checking the Existence of an Item

Instead of the square brackets that allow you to retrieve a value based on its position, you can look for a value based on its complete definition. You have various options. You can first "build" an item and ask the compiler to check whether any item in the list matches your definition. To perform this search, depending on your class, you can call either the ArrayList.Contains() or the List.Contains() method. The syntax of the System.Collections.ArrayList.Contains() method is:

public virtual bool Contains(object value);

The syntax of the System.Collections.Generic.List.Contains() method is:

public bool Contains(T value);

The value to look for is passed as argument to the method. The compiler would look for exactly the value, using its definition, in the list. If any detail of the argument fails to match any value of the list, the method would return false. If all characteristics of the argument correspond to a value of the list, the method returns true. Here is an example:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;

namespace Collector1
{
    public partial class Form1 : Form
    {
        ArrayList lstNames;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            lstNames = new ArrayList();

            lstNames.Add("Christine Kingston");
            lstNames.Add("Hermine Paulson");
            lstNames.Add("William Harrison");
            lstNames.Add("Ursula Morales");
            lstNames.Add("Evan Lancos");

            foreach(string str in lstNames)
                lbxNames.Items.Add(str);
        }

        private void btnResult_Click(object sender, EventArgs e)
        {
            string strFind = txtFind.Text;

            if (lstNames.Contains(strFind) == true)
                txtResult.Text = "Found";
            else
                txtResult.Text = "Not Found";
        }
    }
}
Array List
Array List

Searching for an Item

Another option to look for an item in a list consists of calling the BinarySearch() method of either the ArrayList or the List class. It is overloaded in three versions and one of them uses the following syntax:

public virtual int BinarySearch(object value);
public int BinarySearch(T value);

The value to look for is passed argument to the method. Here is an example:

private void btnResult_Click(object sender, EventArgs e)
{
    string strFind = txtFind.Text;

    if( lstNames.BinarySearch(strFind) > 0 )
        txtResult.Text = "Found";
    else
        txtResult.Text = "Not Found";
}

Clearing a List

To remove all items from a list at once, you can call the Clear() method of either the ArrayList or the List class. Its syntax is:

public virtual void Clear()

Deleting an Item

As opposed to adding a value to a list, you may want to remove one. To perform this operation, you have various options. You can ask the compiler to look for an item in the list and if, or once, the compiler finds it, it would delete the value. To perform this type of deletion, you can call the Remove() method of either the ArrayList or the List class. Its syntax is:

public virtual void Remove(object value);
public bool Remove(T value);

This method accepts as argument the value that you want to delete from the list. To perform this operation, the list must not be read-only.

The Remove() method allows you to specify the exact value you want to delete from a list. Another option you have consists of deleting a value based on its position. This is done using the RemoveAt() method whose syntax is:

public virtual void RemoveAt(int index);
public void RemoveAt(int index);

With this method, the position of the item is passed as argument. Here is an example:

private void btnSecond_Click(object sender, EventArgs e)
{
    lstNames.RemoveAt(1);
    lbxNames.Items.Clear();

    foreach (string str in lstNames)
        lbxNames.Items.Add(str);
}

If the position is not valid because either it is lower or higher than the current Count, the compiler would throw an ArgumentOutOfRangeException exception.

 

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