Asp.net event handler
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Event handling in ASP.NET involves responding to events that occur at the client or server. Events raised at the client are handled at the server. When an event like a button click occurs, the browser posts the event to the server, which checks for an associated event handler method to execute. Common events include page and control loading, data binding, and user interaction events. Event handlers are coded to respond to specific events and take action. Controls generate events that can be handled using attributes and event handler methods.
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Asp.net event handler
- 1. Event handling ASP.net Brought to you by www.SiryMedia.in
- 2. • An event is an action or occurrence such as a mouse click, a key press, mouse movements, or any system-generated notification. A process communicates through events. For example, interrupts are system-generated events. When events occur, the application should be able to respond to it and manage it. • Events in ASP.NET raised at the client machine, and handled at the server machine. For example, a user clicks a button displayed in the browser. A Click event is raised. The browser handles this client-side event by posting it to the server. • The server has a subroutine describing what to do when the event is raised; it is called the event-handler. Therefore, when the event message is transmitted to the server, it checks whether the Click event has an associated event handler. If it has, the event handler is executed.
- 3. Event Arguments • ASP.NET event handlers generally take two parameters and return void. The first parameter represents the object raising the event and the second parameter is event argument. • The general syntax of an event is: • private void EventName (object sender, EventArgs e);Application and Session Events • The most important application events are: • Application_Start - It is raised when the application/website is started. • Application_End - It is raised when the application/website is stopped. • Similarly, the most used Session events are: • Session_Start - It is raised when a user first requests a page from the application. • Session_End - It is raised when the session ends.
- 4. Page and Control Events • Common page and control events are: • DataBinding - It is raised when a control binds to a data source. • Disposed - It is raised when the page or the control is released. • Error - It is a page event, occurs when an unhandled exception is thrown. • Init - It is raised when the page or the control is initialized. • Load - It is raised when the page or a control is loaded. • PreRender - It is raised when the page or the control is to be rendered. • Unload - It is raised when the page or control is unloaded from memory.
- 5. Event Handling Using Controls • All ASP.NET controls are implemented as classes, and they have events which are fired when a user performs a certain action on them. For example, when a user clicks a button the 'Click' event is generated. For handling events, there are in-built attributes and event handlers. Event handler is coded to respond to an event, and take appropriate action on it. • By default, Visual Studio creates an event handler by including a Handles clause on the Sub procedure. This clause names the control and event that the procedure handles. • The ASP tag for a button control: • <asp:Button ID="btnCancel" runat="server" Text="Cancel" /> • The event handler for the Click event: • Protected Sub btnCancel_Click(ByVal sender As Object, ByVal e As System.EventArgs) • Handles btnCancel.Click • End Sub
- 6. • An event can also be coded without Handles clause. Then, the handler must be named according to the appropriate event attribute of the control. • The ASP tag for a button control: • <asp:Button ID="btnCancel" runat="server" Text="Cancel" Onclick="btnCancel_Click" /> • The event handler for the Click event: • Protected Sub btnCancel_Click(ByVal sender As Object, ByVal e As System.EventArgs) End Sub
- 7. The common control events are: Event Attribute Controls Click OnClick Button, image button, link button, image map Command OnCommand Button, image button, link button TextChanged OnTextChanged Text box SelectedIndexChanged OnSelectedIndexChanged Drop-down list, list box, radio button list, check box list. CheckedChanged OnCheckedChanged Check box, radio button
- 8. • Some events cause the form to be posted back to the server immediately, these are called the postback events. For example, the click event such as, Button.Click. • Some events are not posted back to the server immediately, these are called non-postback events. • For example, the change events or selection events such as TextBox.TextChanged or CheckBox.CheckedChanged. The nonpostback events could be made to post back immediately by setting their AutoPostBack property to true.
- 9. Default Events • The default event for the Page object is Load event. Similarly, every control has a default event. For example, default event for the button control is the Click event. • The default event handler could be created in Visual Studio, just by double clicking the control in design view. The following table shows some of the default events for common controls:
- 10. Control Default Event AdRotator AdCreated BulletedList Click Button Click Calender SelectionChanged CheckBox CheckedChanged CheckBoxList SelectedIndexChanged DataGrid SelectedIndexChanged DataList SelectedIndexChanged DropDownList SelectedIndexChanged
- 11. HyperLink Click ImageButton Click ImageMap Click LinkButton Click ListBox SelectedIndexChanged Menu MenuItemClick RadioButton CheckedChanged RadioButtonList SelectedIndexChanged
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