ASP.NET DropDownList Control

DropDownList control is used to give a single select option to the user from multiple listed items. You can specify its height and width in pixel by setting its height and width but you will not be able give mutliple select option to the user.

we can add its option items by directly writing into .aspx page directly or dynamically add at run time or bind through database.

Following are some important properties that are very useful.

SelectedValue	Get the value of the Selected item from the dropdown box.
SelectedIndex	Gets or Sets the index of the selected item in the dropdown box.
SelectedItem	Gets the selected item from the list.
Items	Gets the collection of items from the dropdown box.
DataTextField	Name of the data source field to supply the text of the items. (No need to set when you are adding items directly into .aspx page.)
DataValueField	Name of the data source field to supply the value of the items. (No need to set when you are adding items directly into .aspx page.)
DataSourceID	ID of the datasource component to provide data. (Only used when you have any DataSource component on the page, like SqlDataSource, AccessDataSource etc.)
DataSource	The datasource that populates the items in the dropdown box. (Generally used when you are dynamically generating the items from Database.)
AutoPostBack	true or false. If true, the form is automatically posted back to the server when user changes the dropdown list selection. It will also fireOnSelectedIndexChanged method.
AppendDataBoundItems	true or false. If true, the statically added item (added from .aspx page) is maintained when adding items dynamically (from code behind file) or items are cleared.
OnSelectedIndexChanged	Method name that fires when user changes the selection of the dropdown box. (Fires only when AutoPostBack=true.)

Example:

<asp:DropDownList ID="DropDownList1" runat="server" Height="25px" Width="274px">

            <asp:ListItem Value="1">January</asp:ListItem>

            <asp:ListItem Value="2">February</asp:ListItem>

            <asp:ListItem Value="1">March</asp:ListItem>

</asp:DropDownList>

If you run the web application now, you should see that January, February and March  items shown in the DropDownList.

To add items to the DropDownList at run time using  C# code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ListItem  jan = new ListItem("January", "1");
        ListItem  feb = new ListItem("February", "2");
        ListItem  mar = new ListItem("March", "2");

        DropDownList1.Items.Add(jan);
        DropDownList1.Items.Add(feb);

        DropDownList1.Items.Add(mar);
    }
}

If you want a specific listitem to be selected in the dropdownlist, when the page loads, then set the Selected property of the ListItem object to true. 

The HTML with the Selected property is shown below.

     <asp:DropDownList ID="DropDownList1" runat="server">

        <asp:ListItem Value="1">Male</asp:ListItem>

        <asp:ListItem Value="1" Selected="True">Female</asp:ListItem>

   </asp:DropDownList>

A DropDownList is a collection of ListItem objects.  Below adding items to these controls is also very similar to DropDownList.
1. CheckBoxList
2. RadioButtonList
3. BulletedList
4. ListBox