Properties,
Methods and Events
| Property Name |
Description |
| AutoPostBack |
Boolean. Automatically
posts the form if True. Default is False. |
| DataMember |
Identifies the table
in a data source to bind to the control. |
| DataSource |
Identifies the data
source for the items in the list. |
| DataTextField |
Identifies the field
in the data source to use for the option text. |
| DataValueField |
Indicates the field
in the data source to use for the option values. |
| DataTextFormatString |
Format string for
determining how the DataTextField is displayed. |
| Items |
Collection of items
in the control. |
| SelectedIndex |
Indicates the index
number of the selected option. |
| SelectedItem |
The selected item
(duuh). |
| |
|
| Method Name |
Description |
| DataBind |
Binds the control
to its data source. This means the items from the data source
are loaded into the controls ListItem collection. |
| OnSelectedIndexChanged |
Raises the SelectedIndexChanged
event. |
| |
|
| Event Name |
Description |
| SelectedIndexChanged |
The event is raised
whenever a new option is selected. |
How To Get Options Into the DropDownList Control
1. If you are using Visual Studio, drag the DropDownList control
from the toolbox onto the web form. Make sure you choose the control
from the WebForms tab and not the HTML tab. After positioning the
control where you want, resizing it and so on, you can manually enter
the items into the list using Visual Studio. Go to the properties
dialog for the control. If the properties dialog is not visible, right
click on the control and select PROPERTIES from the menu. Find the
entry for "Items" and click on the ellipsis button to bring up the
ListItem Collection Editor. Click the ADD button on the left side
of the editor. On the right side, enter "Georgia" in the text field
and "GA" in the value field. Now click ADD again and enter "Florida"
in the text field and "FL" in the value field. Click ADD again and
enter "Alabama" in the text field and "AL" in the value field. Click
the OK button. Now you have a DropDownList control with three list
items. The text portion of each item will display the state name.
The corresponding value will be the abbreviation of the selected state.
2. You can access the text and value properties of the
selected item in code as follows:
Dim strVariable as String
strVariable = DropDownList1.SelectedItem.Text - To access the text portion
strVariable = DropDownList1.SelectedItem.Value - To access the value portion
3. If you
are not using Visual Studio or if you just allergic to drag 'n drop,
you can code the above directly following the following syntax:
<asp:DropDownList id="DropDownList1" runat="server">
<asp:ListItem Value="GA"> Georgia </asp:ListItem>
<asp:ListItem Value="FL"> Florida </asp:ListItem>
<asp:ListItem Value="AL"> Alabama </asp:ListItem>
</asp:DropDownList>
4. You can write code to add your items also. Here is an example
of how you might populate the control when the page loads for the
first time. Notice, however, that we are only filling in the text
portion of each option, not the value portion.
If Not Page.IsPostBack Then
DropDownList1.Items.Add("Georgia")
DropDownList1.Items.Add("Florida")
DropDownList1.Items.Add("Alabama")
End If
5. You can bind the DropDownList control to an ArrayList. Again,
notice that we are not getting the value portion of each option.
Dim colArrayList as New System.Collections.ArrayList()
If Not Page.IsPostBack Then
colArrayList.Add("Georgia")
colArrayList.Add("Florida")
colArrayList.Add("Alabama")
DropDownList1.DataSource = colArrayList
DropDownList1.DataBind()
End If
6. The following example shows how to populate
the control using a Hashtable. Now we can get both the text and value
portions populated.
Dim myHashTable as new System.Collections.Hashtable()
myHashTable("GA") = "Georgia"
myHashTable("FL") = "Florida"
myHashTable("AL") = "Alabama"
For each Item in myHashTable
Dim newListItem as new ListItem()
newListItem.Text = Item.Value
newListItem.Value = Item.Key
DropDownList1.Items.Add(newListItem)
Next
7. A SortedList is a collection that
stores key/value pairs (like a hashtable) but where the items are
automatically sorted according to key/value. Here is an example:
Dim mySortedList as new System.Collections.SortedList
Dim Item as DictionaryEntry
mySortedList("GA") = "Georgia"
mySortedList("FL") = "Florida"
mySortedList("AL") = "Alabama"
For each Item in mySortedList
Dim newListItem as new ListItem()
newListItem.Text = Item.Value
newListItem.Value = Item.Key
DropDownList1.Items.Add(newListItem)
Next
8. Here is an example of how you might populate the control
from a SQL Server database using a data reader object:
' Let's assume the connection string is stored in a session variable.
Dim strConnect as Strng
strConnect = Session("ConnectionString")
' Open the Connection
Dim Con as new System.Data.SQLClient.SQLConnection(strConnect)
Con.Open()
' SQL Statement
Dim strSQL as String
strSQL = "SELECT State_Name, State_Code FROM TableState ORDER BY State_Name"
' Command, Data Reader
Dim Com as new System.Data.SQLClient.SQLCommand(strSQL, Con)
Dim rdr as System.Data.SQLClient.SQLDataReader = Com.ExecuteReader()
' Populate the Control
While rdr.Read()
Dim newListItem as new ListItem()
newListItem.Text = rdr.GetString(0)
newListItem.Value = rdr.GetString(1)
DropDownList1.Items.Add(newListItem)
End While
Conclusion
I certainly didn't exhaust all the possibilities but I think this
provides a pretty decent overview of the control, how to use it and
how to get data into it. If you have any comments, corrections or
other ideas, you can email me at:
RogerMcCook@usermail.com
About the Author:
Visit Roger McCook's Web site at http://www.mccooksoftware.com.
Read this newsletter at: http://www.webproasp.com/2003/1030.html |
|
Roger
McCook