ASP.NET.in.a.Nutshell.Second.Edition [Electronic resources] نسخه متنی

10.3 Tracing

The other
useful tool provided by ASP.NET for debugging is the trace feature.
Tracing allows you, through a simple configuration setting or
page-level attribute, to have ASP.NET write a whole host of
information about the currently executing request to the page or to a
trace log. This information includes the SessionID; the time, type,
and status code of the request; timing information for events such as
Init, PreRender, SaveViewState, and Render; a Control Tree of all
controls in the page; and the contents of the Cookies collection, the
HTTP Headers, the QueryString collection (if any QueryString values
were passed), the Form collection (if any form fields were passed),
and the ServerVariables collection.

Essentially, tracing allows you to automatically write out the
contents of all collections exposed by the classic ASP Request
object, plus some really useful additional information. This allows
you to examine a great deal of information about a request on a
single page, which can assist greatly in debugging.

More importantly, you can also write to the trace output using the
Trace.Write and Trace.Warn methods, which are exposed by the Trace
property of the Page class. These methods can now
be used in place of Response.Write when you determine the value of a
variable on a page at a certain point in page execution or write out
a notification that a certain point in the page has been hit. With
Trace.Write or Trace.Warn, once you've disabled
tracing, you can leave the statements in your code and you
don't have to worry about anyone seeing the output.
If you've ever deployed a page to production without
removing or commenting out your Response.Write statements used for
debugging, you'll be grateful for this feature.

10.3.1 Page-Level Tracing

Enabling tracing at the page level is very
simple. Simply add the Trace attribute to the
@
Page directive and set its value to
True, as shown in the following code snippet:

<%@ Page Language="VB" Trace="True" %>

This provides a quick and easy way to get an overview of
what's going on for a given page. The output from a
page with tracing enabled looks similar to Figure 10-8.

Figure 10-8. Page-level tracing

One downside to enabling tracing at the page level is that if you use
it with many pages, it's more work to add and remove
the Trace attribute for each page (or to set the
attributes to False). This can also make it more
likely that you'll forget one and end up with trace
information being written out in a production application. Not what
you want.

10.3.2 Application-Level Tracing

ASP.NET provides the ability to enable tracing
at the application level through the
<trace> element in

web.config .
Application-level tracing makes it possible to disable tracing in a
single location in your application and makes it easier to enable
tracing for multiple pages. Example 10-8 shows a

web.config file with tracing enabled at the
application level.

While the <trace> element allows you to enable or disable tracing at the application level, the page-level Trace attribute overrides the setting in web.config . Thus, if any pages have the Trace attribute set to True, disabling tracing at the application level will not disable tracing for these pages.

Example 10-8. The <trace> element

<configuration>
<system.web>
<trace enabled="true"
localOnly="true" 
pageOutput="true"
requestLimit="15"
traceMode="SortByCategory" />
<system.web>
</configuration>

In addition to providing a single point of control over
enabling/disabling tracing, the <trace>
element allows you to control several other factors: whether trace
output is visible to machines other than the local host (using the
localOnly attribute); whether the output is sent
to the page or to a trace log (using the
pageOutput attribute); the number of traces that
are kept in the trace log (requestLimit); and how
the Trace Information section of the trace output is sorted
(traceMode).

If pageOutput is set to False,
you can still view the trace output by entering the special URL

Trace.axd (which
isn't an actual file, but a URL that invokes an
HttpHandler for the trace functionality) from the root of the
application.

Trace.axd lists all saved traces
for the application. Note that once the number of traces specified by
the requestLimit attribute is met, no more traces
are saved until the trace log is cleared.

Trace.axd provides a link for this purpose.

10.3.3 Using Trace.Write and Trace.Warn

Finally, as mentioned in the introduction to this
section, instead of using Response.Write (as done in classic ASP) to
write out variable values or flag certain points in your code,
ASP.NET enables you to use

Trace.Write and

Trace.Warn . Both the Write and Warn methods are overloaded
and can take one of the following sets of arguments:

• A single string argument containing the text to write to the trace
  output.

• One string argument containing a user-defined category for the entry
  and a second string argument containing the text to write to the
  trace output.

• One string argument containing a user-defined category for the entry,
  a second string argument containing the text to write to the trace
  output, and an Exception argument.

The only difference between Write and Warn is that entries written
with the Warn method appear in red text, making them ideal for
conditions that require special attention. Example 10-9 shows an ASP.NET page with tracing enabled
that uses Trace.Write and Trace.Warn to write to the trace output.
Figure 10-9 shows the output of this page.

Example 10-9. Trace.aspx

<%@ Page Language="VB" Trace="True" %>
<html>
<head>
<title>Tracing Sample</title>
<script runat="server">
Sub Page_Load( )
If Page.Trace.IsEnabled = True Then
Trace.Write("MyCategory", "Hello, Trace!")
Trace.Warn("MyCategory", "This text will be red!")
Message.Text = "Tracing is enabled for this page."
Else
Message.Text = "Tracing is not enabled for this page."
End If
End Sub
</script>
</head>
<body>
<asp:label id="Message" runat="server"/>
</body>
</html>

Figure 10-9. Output of Trace.aspx