Beginning Web Development, Silverlight, and ASP.NET AJAX

CHAPTER 3 ■ WEB FORMS WITH ASP.NET 49
}
}
}
int nR = 0;
if (sOperation == "Add")
nR = nF + nS;
else
nR = nF - nS;
litResult.Text = nR.ToString();
Now when you run your application, enter values into the text boxes, and click the
submit button, you’ll see that the value of the operation is loaded into the Literal control
and displayed on the page. But the text boxes are then emptied! Why is this
The same reasoning that applied to the ASP.NET HTML control for the mathematical
results also applies to the text boxes containing the values to be added or subtracted.
ASP.NET is unaware that your intention was to maintain the control’s view state, and
indeed, when the page posted back, ASP.NET very efficiently threw the values to be
added or subtracted away once the page was refreshed.
As before, one solution is to add the runat and id attributes to these controls. And
that works. But the best solution to this problem is to use the full-fledged ASP.NET
TextBox controls and have ASP.NET manage their view state for you. Remember, ASP.NET
converts the TextBox controls into HTML input text boxes for you, but the view state
functionality ASP.NET employs can now be used to maintain their contents as the request
flow passes back and forth. The next step is to replace the ASP.NET HTML text input controls
with ASP.NET TextBox controls. You can find these on Visual Studio’s Standard
Toolbox tab alongside the Literal control.
Listing 3-7 shows the revised code for the Default.aspx page using controls
instead of HTML controls for the text boxes.
Listing 3-7. Modified Listing for ASP.NET Text Boxes
Untitled Page