by Troy Karhoff
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| Introduction |
This article covers a fundamental use of the .NET LinkButton web control for page postbacks.
A web developer is routinely faced with the challenge of knowing which link was clicked. The typical solution involves passing information on the querystring in the clear, like this: "GetProduct.asp?ID=3762". A key benefit of the LinkButton web control is the power to know which link was clicked by using POST rather than GET. This is accomplished by setting 2 properties on the LinkButton: CommandName and CommandArgument. These properties are passed into the onCommand event when the user clicks a LinkButton. In the event handler for onCommand, the CommandName and CommandArgument properties can be evaluated in CommandEventArgs. Because this all takes place in the POST, the querystring is shorter and the postback data is hidden. |
| Mini databases |
Let's begin by looking at some code. First, a "mini" database of products is created in the code using two instances of System.Collection.Hashtable. One hashtable stores the ProductId and ProductTitle, and the other hashtable stores the ProductId and ProductDescription. The two Hashtables are related by the ProductId. This technique is OK for demonstration purposes; however, there are cleaner ways to accomplish this.
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Hashtable hashProductName = new Hashtable();
Hashtable hashProductDesc = new Hashtable();
void BuildMiniDatabase()
{
hashProductName[0] = "Jalapeno Dip" ;
hashProductDesc[0] = "Simmered in mayonaise and wine, this Jalapeno Dip will make your eyes water" ;
hashProductName[1] = "Smoked Sausage" ;
hashProductDesc[1] = "Mouth watering and delicious sausage" ;
hashProductName[2] = "Shrimp Fiesta" ;
hashProductDesc[2] = "East Coast's finest shrimp" ;
hashProductName[3] = "Jerk Chicken" ;
hashProductDesc[3] = "A real island experience you will not forget" ;
hashProductName[4] = "Beer-Battered Fish" ;
hashProductDesc[4] = "Pabst Blue Ribbon and Fish. Wow!" ;
hashProductName[5] = "Bacon Burger" ;
hashProductDesc[5] = "Big, juicy, and bursting with flavor" ;
hashProductName[6] = "Sirloin Tip" ;
hashProductDesc[6] = "Delicate cuts with no fat" ;
hashProductName[7] = "Baked Alaska" ;
hashProductDesc[7] = "Fine dessert comprised of sponge cake topped with ice cream and covered with meringue. The meringue is browned before the ice cream can melt." ;
hashProductName[8] = "Fried Chicken" ;
hashProductDesc[8] = "Country cookin'" ;
hashProductName[9] = "Fresh Garden Salad" ;
hashProductDesc[9] = "Crispy iceberg lettuce and a garden of vegtables" ;
hashProductName[10] = "One Pea" ;
hashProductDesc[10] = "A single green pea that will leave you craving more" ;
}
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| Web page layout |
Next, the web page layout is very basic. Inside a form, a table is built with 2 columns. The left side cell will hold the product links. The right cell will display the product description.
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<body>
<form id="MainForm" method="post" runat="server" >
<asp:Table CellPadding=6 CellSpacing=2 BorderColor="#DDDDDD" BorderStyle=Solid BorderWidth=2 Runat=server>
<asp:TableRow Runat=server>
<asp:TableCell id=LinkList Wrap=False BackColor="#FFFFFF" Runat=server/>
<asp:TableCell id="tablecellMessage" CssClass="ProductDesc" Runat=server></asp:TableCell>
</asp:TableRow>
</asp:Table>
</form>
</body>
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| Adding LinkButtons |
Now that the web page is designed, the LinkButtons can be added. The LinkButtons are added dynamically using code. The System.Web.UI.WebControls.LinkButton control is instantiated and assigned to _LB1. The Text property is used for rendering the hyperlink label. That is, the text that sits between <a></a>. The CommandName and CommandArgument are what's it all about. Use the CommandArgument property to specify an argument that complements the CommandName property. What might have been placed on the querystring, will now simply be assigned in the CommandName and/or CommandArgument properties. In my example, I'm really just using the CommandArgument property for storing the ProductId of each product link. The CommandName is used to identify my grouping of links which happen to be the same - they're all products.
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void BuildLinkList()
{
for (int i=1; i<=10; i++)
{
LinkButton _LB1 = new LinkButton();
_LB1.Text = hashProductName[i].ToString();
_LB1.CssClass = "ProductLinks";
_LB1.CommandName = "Products";
_LB1.CommandArgument = i.ToString() ; | |
| Adding onCommand event and OnLinkClick event handler |
The onCommand event is fired when a user clicks a LinkButton. Because my LinkButtons are added programatically, I need to wire the onCommand event to the control and assign it to the OnLinkClick event handler.
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_LB1.Command += new System.Web.UI.WebControls.CommandEventHandler(OnLinkClick);
LinkList.Controls.Add(_LB1);
LinkList.Controls.Add(new LiteralControl("<br>")); |
When the user clicks a product link, the OnLinkClick event handler will determine which link was clicked, get the ProductId from the CommandArgument property, retrieve the ProductDescription from the mini database, and display the result on the web page.
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void OnLinkClick(object O, System.Web.UI.WebControls.CommandEventArgs E)
{
int RecordId = Int32.Parse(E.CommandArgument.ToString());
tablecellMessage.Text = "<b>" + hashProductName[RecordId].ToString() + "</b><br><i>" + hashProductDesc[RecordId].ToString() + "</i>";
} |
The LinkButton web control will most likely become a fundamental part of your ASP.Net programming. |
| Conclusion |
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