Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

this effect could have been achieved just as easily through CSS, but in future,

when you’re working on projects that utilize more complex controls and properties,

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

use them.

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

Take a look at this code:

Visual Basic File: ErrorTest.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# File: ErrorTest.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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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 eleventh element, which doesn’t exist.

If you load this page directly in Internet Explorer without debugging it, you’ll

see a page that specifies the error, like the one shown in Figure 5.46.

Figure 5.46. The error message isn’t very helpful without debug

mode

You can obtain more details by enabling debug mode and, if you scroll down,

you’ll see instructions that explain how to do just that. The easiest way to enable

debug mode it to use Visual Web Developer. You’ll remember from earlier in

this chapter that the first time you execute a page by pressing F5 (Start Debug-

ging), Visual Web Developer asks you if you want it to enable debug mode for

you. If you ask it to, it’ll modify (or create) the Web.config file accordingly.

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Debugging with Visual Web Developer

Figure 5.47. 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 5.47 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 mes-

sage.

In Figure 5.47, 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

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Watch window; in response, it will display their values and types. Try typing

a(5) (or a[5] if you’re using C#) in the Watch window; you should see a display

like the one in Figure 5.48.

Figure 5.48. Inspecting values using the Watch window

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

Figure 5.49.

Figure 5.49. 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 had 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.

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Debugging with Visual Web Developer