What Will I Learn?

• Changing the Contents of a Drop-Down List with C#

Requirements

• SharpDevelop IDE

Difficulty

• Intermediate

Tutorial Contents

• Creating the ComboBox
• Populating (and Repopulating) the ComboBox

How to Create a Dynamic C# ComboBox

Every C# Windows application needs at least one ComboBox - especially when the contents of the ComboBox can be updated dynamically.

A ComboBox is a very useful component in any C# Windows application. It enables a form’s user to select information from a drop-down list. For example, in an application used to arrange delivery of an item the user should be allowed to choose the day of the week.

A ComboBox provides a simple way of doing this. However, in an example such as this there is one key issue: the day of the week changes according to the language being used (the English Monday is Lundi in French and Lunes in Spanish). So, rather than teach every user English the programmer may create a dynamic ComboBox whose contents change according the language being used.

Creating the ComboBox

As well as the ComboBox the user will need some way of selecting the language to be used. Radio buttons provide a suitable way of doing this:

using System;
using System.Windows.Forms;
using System.Drawing;
public class MainForm : Form {
private ComboBox cboDay = new ComboBox();
private RadioButton radBritish = new RadioButton();
private RadioButton radFrench = new RadioButton();
private RadioButton radSpanish = new RadioButton();

The code will also need to contain the days of the week. In this example these are stored in three arrays. These arrays will be used to load the ComboBox according to which radio button is selected:

private string[] britDays = {
"Sunday",
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday"};
private string[] frenchDays = {
"Dimanche",
"Lundi",
"Mardi",
"Mecredi",
"Jeundi",
"Vendredi",
"Samedi"};
private string[] spanishDays = {
"Domingo",
"Lunes",
"Martes",
"Miercoles",
"Jueves",
"Viernes",
"Sabato"};

The ComboBox can then be added to the form:

public static void Main() {
Application.Run(new MainForm());
}
public MainForm() {
cboDay.Location = new Point (0, 50);
this.Controls.Add(cboDay);

As can the radio buttons:

radBritish.Location = new Point(0, 0);
radBritish.Text = "English";
radBritish.AutoSize = true;
this.Controls.Add(radBritish);

Radio buttons for French and Spanish can be created in a similar way, although the coordinates given for the location (in x and y) will have to be different. If the form is compiled at this point then the ComboBox and radio buttons will be visible but will not actually do anything yet. The next stage, therefore, is to populate the ComboBox and assign actions to the radio boxes (so that the ComboBox is updated).

Populating (and Repopulating) the ComboBox

The ComboBox will be populated, cleared and then repopulated with the contents of the three arrays (for English, French and Spanish). Creating a new function will enable this to be done:

public void showDayOfWeek (string[] days) {
cboDay.Items.Clear();
cboDay.Items.AddRange(days);
}

The function clears the ComboBox and then adds the items listed in an array passed to it. The programmer can then use this to set the initial state of the form:

radBritish.Checked = true;
showDayOfWeek(britDays);

This will check the "English" radio button and populate the ComboBox with the list ofEnglish days. The contents of the ComboBox will not actually change yet. For that some more functions are required:

public void radFrench_Click(object sender, EventArgs e) {
showDayOfWeek(frenchDays);
}
public void radBritish_Click(object Sender, EventArgs e) {
showDayOfWeek(britDays);
}
public void radSpanish_Click(object Sender, EventArgs e) {
showDayOfWeek(spanishDays);
}

And then these functions can be assigned to the radio buttons:

radBritish.Click += new EventHandler(radBritish_Click);
radFrench.Click += new EventHandler(radFrench_Click);
radSpanish.Click += new EventHandler(radSpanish_Click);

Now, if the form is compiled, the ComboBox will be populated with the correct days for the language selected.

Summary

Once a programmer has added a new ComboBox object to a C# Windows application it can be populated with the contents of an array by using the "AddRange" method. The programmer can repopulate it by first using the "Clear" method and then reusing "AddRange" with a different array.

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