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Thursday, August 30, 2007

ASP.NET Tip: Control the Layout of Your Input Forms

Although many of your input forms will have a fixed number of fields, in some cases you'll need to vary the number and configuration of your controls. This article demonstrates how to control the layout of your form using a Repeater control and the ItemDataBound event to set various attributes on the controls.

The example form has a simple table that includes the following Repeater control:

<asp:Repeater ID="rptFields" runat="server">
<ItemTemplate>
<tr>
<td align="right" ID="tdPrompt" runat="server"
nowrap><%# Eval("Prompt") %>:</td>
<td><asp:TextBox ID="txtValue" runat="server" />
<input type="hidden" id="hdnFieldID"
value='<%# Eval("pkFieldID") %>'
runat="server" /></td>
</tr>
</ItemTemplate>
</asp:Repeater>

Behind the scenes, a table called FormFields (or something appropriate) has these fields:

pkFieldID
Primary Key, integer

Prompt
String, contains the prompt to prefix the field with

MaxLength
Integer, indicates maximum number of characters for the field

IsRequired
Bit, indicates whether field is required

In the initial Load event for the page, I retrieve a DataTable containing the fields from the database, and then I bind it to the Repeater like this:

rptFields.DataSource = fieldsDataTable;
rptFields.DataBind();
The key event handler to make this work is the ItemDataBound event handler. It looks something like this:

void rptParameters_ItemDataBound(object sender,
RepeaterItemEventArgs e)
{
if (e.Item.ItemType != ListItemType.Item && e.Item.ItemType
!= ListItemType.AlternatingItem)
return;

DataRow dr = ((DataRowView)e.Item.DataItem).Row;
HtmlTableCell td = (HtmlTableCell)e.Item.FindControl("tdPrompt");

if (Convert.ToBoolean(dr["IsRequired"]))
td.Attributes["class"] = "popupreq";
else
td.Attributes["class"] = "popuptext";

TextBox txt = (TextBox)e.Item.FindControl("txtValue");
txt.MaxLength = Convert.ToInt32(dr["MaxLength"]);

}

The first thing you do is make sure that you are looking at an "Item" or "Alternating Item" template. This is because the ItemDataBound event handler is called for the Header and Footer templates, which you don't care about for this code. You then get the DataRow for the current item being bound. Next, you find the table cell that contains the prompt text and change the style to either 'popupreq' or 'popuptext'. In this example, the popupreq style uses bold face, while popuptext uses a non-bold font.

The next step is to find the TextBox control and configure the maximum length of the field. The hidden field in the HTML portion of the page will be populated automatically with the primary key of the field record, which will be used in the back-end code to properly store the data to the database.

When the user hits the Submit button, ASP.NET keeps the values that the user enters into each field and makes them available to the PostBack code. Reading the values is similar to setting them in ItemDataBound, as this snippet shows:

foreach (RepeaterItem ri in rptParameters.Items)
{
hdn = (HtmlInputHidden)ri.FindControl("hdnFieldID");
txt = (TextBox)ri.FindControl("txtValue");
// Read hdn.Value or txt.Text for persistence code
}

The reason you stored the field ID is because this data will generally need to be put into a table related to the data being edited. The easiest way I've found to do this is to fill the hidden field with the field's number, which you can use later in a stored procedure or other method to save the field's data to the database.

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