How to: Add Controls to an ASP.NET Web Page Programmatically

  • Makale

At times it is more practical to create a control at run time than at design time. For example, imagine a search results page in which you want to display results in a table. Because you do not know how many items will be returned, you want to dynamically generate one table row for each returned item.

Note

Existing controls can often provide the functionality you get from creating controls dynamically. For example, controls such as the Repeater, DataList, and RadioButtonList controls can dynamically create rows or other control elements when the page runs.

In order to programmatically add a control to a page, there must be a container for the new control. For example, if you are creating table rows, the container is the table. If there is no obvious control to act as container, you can use a PlaceHolder or Panel Web server control.

In some instances, you might want to create both static text and controls. To create static text, you can use either a Literal or a Label Web server control. You can then add these controls to the container as you would any other control. For information about view state in controls created at run time, see Dynamic Web Server Controls and View State.

To add a control to an ASP.NET Web page programmatically

  1. Create an instance of the control and set its properties, as shown in the following example:

    Dim myLabel As New Label()
myLabel.Text = "Sample Label"

    Label myLabel = new Label();
myLabel.Text = "Sample Label";

    Note

    Controls are typically added to the page during the page's initialization stage. For details about page stages, see ASP.NET Page Life Cycle Overview.

  2. Add the new control to the Controls collection of a container already on the page, as shown in the following example:

    Dim Panel1 As New Panel()
Panel1.Controls.Add(myLabel)

    Panel Panel1= new Panel();
Panel1.Controls.Add(myLabel);

    Note

    Because the Controls property is a collection, you can use the AddAt method to place the new control at a specific location — for example, in front of other controls. However, this can introduce errors into the page. For details, see Dynamic Web Server Controls and View State.

    The following code example shows the event handler for the SelectedIndexChanged event of a control named DropDownList1. The handler creates as many label controls as the user has selected from the drop-down list. The container for the controls is a PlaceHolder Web server control named Placeholder1.

    Security noteSecurity Note:

    User input in a Web page can include potentially malicious client script. By default, ASP.NET Web pages validate that user input does not include script or HTML elements. For more information, Script Exploits Overview.

    		 
    Private Sub DropDownList1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
    Dim DropDownList1 As New DropDownList()
    Dim PlaceHolder1 As New PlaceHolder()
    Dim i As Integer
    Dim numlabels As Integer

    ' Get the number of labels to create.
    numlabels = CInt(DropDownList1.SelectedItem.Text)
    For i = 1 To numlabels
        Dim myLabel As Label = New Label()
        ' Set the label's Text and ID properties.
        myLabel.Text = "Label " & i
        myLabel.ID = "Label" & i
        PlaceHolder1.Controls.Add(myLabel)
        ' Add a spacer in the form of an HTML <br /> element
        Dim spacer As LiteralControl = New LiteralControl("<br />")
        PlaceHolder1.Controls.Add(spacer)
    Next
End Sub

    private void DropDownList1_SelectedIndexChanged(object sender, System.EventArgs e)
{
    DropDownList DropDownList1 = new DropDownList();
    PlaceHolder PlaceHolder1 = new PlaceHolder();

  // Get the number of labels to create.
 int numlabels = System.Convert.ToInt32(DropDownList1.SelectedItem.Text);
 for (int i=1; i<=numlabels; i++)
 {
   Label myLabel = new Label();

   // Set the label's Text and ID properties.
   myLabel.Text = "Label" + i.ToString();
   myLabel.ID = "Label" + i.ToString();
   PlaceHolder1.Controls.Add(myLabel);
   // Add a spacer in the form of an HTML <br /> element.
   PlaceHolder1.Controls.Add(new LiteralControl("<br />"));
 } 
}

See Also

Tasks

How to: Set HTML Server Control Properties Programmatically

How to: Set ASP.NET Server Control Style Properties Programmatically

Concepts

ASP.NET Web Server Control Event Model

Other Resources

Setting ASP.NET Server Control Properties Programmatically