Every computer programmer has run into bugs. It comes with the territory. Many bugs are found during the coding process. Others pop up only when an end user performs a specific and unusual sequence of steps or the program receives unexpected data. It is highly desirable to find bugs early in the development process, and very important to avoid having end users find your bugs for you. Countless studies have shown that the earlier you find a bug, the easier and less expensive it is to fix.
In the event that your program does run into a problem, you will want to recover quickly and invisibly, or, at worst, fail gracefully. ASP.NET provides tools and features to help reach these goals, including:
- Tracing
You can easily trace program execution at either the page or application level. ASP.NET provides an extensible trace log with program lifecycle information.
- Symbolic debugging
You can step through your program, set breakpoints, examine and modify variables and expressions, and step into and out of classes, even those written in other languages.
- Error handling
You may handle standard or custom errors at either the application or page level. You can also show different error pages for different errors.
To get started exploring the ASP.NET debugging tools, you should first create a simple project to which you will add tracing code. You will then introduce bugs into the program and use the debugger to find and fix the bugs.
To start, create a new C# or VB.NET web application project in Visual Studio .NET and name it DebuggingApp. This project will consist of a single web page containing a header label, a DropDownList with a label below it to display the selected item, and a hyperlink.
Change the pageLayout property of the document from the default GridLayout to FlowLayout. Put a Label control on the top of the page and set the text (its Text property) to:
Tracing, Debugging & Error Handling Demo
Change its Font.Name property to Arial Black, its Font.Size property
to Large, and its Font.Bold property to true.
Place a DropDownList control on the form. Name it ddlBooks. Change
its AutoPostBack property to true. The drop-down
list’s event handling code needs to be added. The
exact steps for doing so are different in C# than in VB.NET.
In C#, click on the Events icon at the top of the Properties box, click on the field next to SelectedIndexChanged, and type in the event handler name ddlBooks_SelectedIndexChanged.
In VB.NET, display the code-behind source code window by clicking on the code-behind file tab at the top of the source code window. At the top of the window are two drop-down lists. The list on the left contains all the objects on the page. Select ddlBooks from that list. The drop-down on the right will contain all the available events. Select SelectedIndexChanged. Alternatively, double-click on the control in the Design view of the .aspx file. In either case, this will immediately open the code-behind page with the cursor inside the event handler method with that name, ready for you to add the event handling code.
Type or paste in the code in Example 7-1, if you’re programming in C#, or Example 7-2, if you’re programming in VB.NET. These examples are excerpted from Example 5-25 and Example 5-26, respectively.
Add the code shown in Example 7-3, if you’re using C#, or Example 7-4, if you’re programming in VB, to replace the Page_Load event. (Again, these examples are excerpted from Example 5-25 and Example 5-26, respectively.)
Example 7-3. Page_Load event handler in C#
private void Page_Load(object sender, System.EventArgs e)
{
// Put user code to initialize the page here
if (! IsPostBack)
{
// Build 2 dimensional array for the lists
// First dimension contains bookname
// 2nd dimension contains ISBN number
string[,] books = {
{"Programming C#","0596001177"},
{"Programming ASP.NET","1234567890"},
{"WebClasses From Scratch","0789721260"},
{"Teach Yourself C++ in 21 Days","067232072X"},
{"Teach Yourself C++ in 10 Minutes","067231603X"},
{"XML & Java From Scratch","0789724766"},
{"Complete Idiot's Guide to a Career in Computer Programming","0789719959"},
{"XML Web Documents From Scratch","0789723166"},
{"Clouds To Code","1861000952"},
{"C++: An Introduction to Programming","1575760614"},
{"C++ Unleashed","0672312395"}
};
// Now populate the lists.
int i;
for (i = 0; i < books.GetLength(0); i++)
{
// Add both Text and Value
ddlBooks.Items.Add(new ListItem(books[i,0],books[i,1]));
}
}
}Example 7-4. Page_Load event handler in VB.NET
Private Sub Page_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
'Put user code to initialize the page here
if not IsPostBack then
' Build 2 dimensional array for the lists
' First dimension contains bookname
' 2nd dimension contains ISBN number
dim books(,) as string = { _
{"Programming C#","0596001177"}, _
{"Programming ASP.NET","1234567890"}, _
{"WebClasses From Scratch","0789721260"}, _
{"Teach Yourself C++ in 21 Days","067232072X"}, _
{"Teach Yourself C++ in 10 Minutes","067231603X"}, _
{"XML & Java From Scratch","0789724766"}, _
{"Complete Idiot's Guide to a Career in Computer Programming", _
"0789719959"}, _
{"XML Web Documents From Scratch","0789723166"}, _
{"Clouds To Code","1861000952"}, _
{"C++: An Introduction to Programming","1575760614"}, _
{"C++ Unleashed","0672312395"} _
}
' Now populate the lists.
dim i as integer
for i = 0 to books.GetLength(0) - 1
' Add both Text and Value
ddlBooks.Items.Add(new ListItem(books(i,0),books(i,1)))
next
end if
End SubAdd a label below the DropDownList called lblDdl. Set the Text property so that it is empty.
Finally, add a HyperLink control below lblDdl. Name it hplTest.
Change the Text property to Link To and change the NavigateUrl
property to TestLink.aspx. Note that no page
with this name actually exists. This is an intentional error to
demonstrate error handling later in the chapter.
Run the web page and select one of the items in the drop-down list; you should see something like Figure 7-1.
