Build Your Own ASP.NET 3.5 Website Using C# & VB (39 page)

be your only choice. As such, it’s important that you know how to use them.

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Debugging and Error Handling

Your work with Dorknozzle for this chapter is over, but now that we’ve started to

create a real-world application, it’s time to consider the real-world problems that

might occur as we’re developing that application. A constant truth in the life of any

programmer is that programming mistakes do happen, and they happen no matter

how experienced the programmer is. For this reason, it’s beneficial to know what

you can do when you encounter an error, and to learn how ASP.NET and Visual

Web Developer can help you analyze and debug your code.

Debugging with Visual Web Developer

Create a new web form in Visual Web Developer that uses a code-behind file in

your
C:\LearningASP\VB
or
C:\LearningASP\CS
folder, and call it
ErrorTest.aspx
. Then add the code below to the Page_Load method (remember, if you’re using VB, to

double-click the page in design mode to generate the method) in the code-behind

file:

Visual Basic

LearningASP\VB\ErrorTest_01.aspx.vb
(excerpt)

Protected Sub Page_Load(ByVal sender As Object,

➥ ByVal e As System.EventArgs) Handles Me.Load

Dim a(10) As Integer

Dim i As Integer

For i = 1 To 11

a(i) = i

Next

End Sub

C#

LearningASP\CS\ErrorTest_01.aspx.cs
(excerpt)

protected void Page_Load(object sender, EventArgs e)

{

int[] a = new int[10];

int i;

for (i = 0; i < 11; i++)

{

a[i] = i;

}

}

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Building Web Applications

213

The code above creates an array of ten elements, then uses a For loop to assign

values to them. The problem is that it doesn’t stop at the tenth element: it also tries

to assign a value to the 11th element, which doesn’t exist.

If you execute the page without debugging (using the
CTRL+F5
shortcut), the page

will generate an error in your web browser
, as shown in Figure 5.35.

Figure 5.35. Viewing the unhelpful error message

You can obtain more details by executing the project in debug mode (by pressing

F5
in Visual Web Developer).

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Figure 5.36. Debugging a run-time error

Executing the page once again—this time, with debugging enabled—takes you

straight to the error in Visual Web Developer
, as Figure 5.36
illustrates. This interface tells you that the code has thrown an
exception
of type

IndexOutOfRangeException. In .NET, exceptions are the standard means by which

errors are generated and propagated. An exception is a .NET class (in this case, the

IndexOutOfRangeException class) that contains the details of an error. As you’ll

see a little later, you can
catch
the error in your code using the Try-Catch-Finally

construct. If the error isn’t caught and handled, as in this case, it’s finally caught

by the ASP.NET runtime, which generates an error message.

In Figure 5.36
, the debugger has paused execution at the moment the exception was raised. Let’s see what your options are at this moment. One very useful window is

the Watch window, which appears by default when your application is being debugged. If it’s not displayed, you can open it by accessing
Debug
>
Windows
>
Watch
. You can type the names of the objects in your code into the Watch window; in response, it will display their values and types. Try typing
a(5)
(or
a[5]
if you’re using C#) into the W
atch window; you should see a display like the one in Fig-

ure 5.37.

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215

Figure 5.37. Inspecting values using the Watch window

You could even type just
a
, then explore its members via the display shown in

Figure 5.38.

Figure 5.38. The Watch window showing the contents of an array

Arrays and VB

This example reveals an interesting aspect of this array. The Watch window reports

that the array’s length is 11, yet we defined it as a(10). In all .NET languages,

arrays are zero-based, which means that the first element of an array is a(0), the

second is a(1), and so on. So an array called a that had ten elements would have

as its first element a(0), and a(9) as its last.

However, VB offers extra assistance for developers who are experienced with pre.NET versions of the language (which used one-based arrays in which the first element would have been a(1), and the last would have been a(10)): it adds an

element for you. In other words, if you declare an array of ten elements in VB,

you’ll get an array of 11 elements.

C# has always had zero-based arrays, so an array defined as a[10] will have ten

elements.

In more complex scenarios, if you enter the name of an object, the Watch window

will let you explore its members as we just saw.

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If you switch to the Locals window (
Debug
>
Windows
>
Locals
) shown in Figure 5.39
, you can see the variables or objects that are visible from the line of code at which

the execution was paused.

Figure 5.39. The Locals window

Another nice feature of Visual Web Developer is that when you hover your cursor

over a variable, the editing window shows you at-a-glance information about that

variable.

Sometimes, you’ll want to debug your application even if it doesn’t generate an

exception. For example, you may find that your code isn’t generating the output

you expected. In such cases, it makes sense to execute pieces of code line by line,

and see in detail what happens at each step.

The most common way to get started with this kind of debugging is to set a
break-

point
in the code. In Visual Web Developer, we do this by clicking on the gray bar

on the left-hand side of the editing window. When we click there, a red bullet appears, and the line is highlighted with red to indicate that it’
s a breakpoint, as Fig-

ure 5.40 illustrates.

Once the breakpoint is set, we execute the code. When the execution pointer reaches

the line you selected, execution of the page will be paused and Visual Web Developer

will open your page in debug mode. In debug mode, you can perform a number of

tasks:

■ View the values of your variables or objects.