The CheckBox control is a very common control of HTML, unlike radio buttons it can select multiple items on a webpage. The CheckBox control in ASP.NET has many properties and some of them are listed below.
Property |
Description |
AutoPostBack |
Specifies whether the form should be posted immediately after the Checked property has changed or not. The default is false. |
CausesValidation |
Specifies if a page is validated when a Button control is clicked. |
Checked |
Specifies whether the check box is checked or not. |
InputAttributes |
Attribute names and values used for the Input element for the CheckBox control. |
LabelAttributes |
Attribute names and values used for the Label element for the CheckBox control. |
runat |
Specifies that the control is a server control. Must be set to "server". |
Text |
The text next to the check box. |
TextAlign |
On which side of the check box the text should appear (right or left). |
ValidationGroup |
Group of controls for which the Checkbox control causes validation when it posts back to the server. |
OnCheckedChanged |
The name of the function to be executed when the Checked property has changed. |
In this article I will teach you how to limit the number of selected items among all the checkboxes on a webpage. The first step is to create an "ASP.NET Empty Web Application".
Since we have started with an Empty Web Application we need to add a page now. Go to Project > Add New Item (Ctrl+Shift+A); a screen will pop up; add a web form from it and name it "SelectionLimit.aspx".
The explorer will have SelectionLimit.aspx open for you with some default code. Add a Check Box List either from a toolbox or write the code. Since we need to limit the number of selections we need to call a function or submit our form every time a selection is made in the check box list. For this we will use the property AutoPostBack; set it to "true" since we need to call a function on every Post Back to trigger an event when the selection changes. To do this go to the properties on the bottom-right corner.
As soon as you double-click the SelectedIndexChanged, "SelectionLimit.aspx.cs" will show up where you will write code.
We will have the following two functions there:
protected void Page_Load(object sender, EventArgs e), and
protected void MyCheckBoxList_SelectedIndexChanged(object sender, EventArgs e)
Since we have just added CheckBoxList and not added any items to it, so in the "Page_Load" Function we will add items to the CheckBoxList:
And in the "MyCheckBoxList_SelectedIndexChanged" function we will do the main coding part:
That is all. Debug your page, you might have noticed in the "if" condition that I used count > 2 i.e. I'm limiting the selection to two selections only i.e. only two items can be selected.