Display tracing information

1. Open Visual Studio and create a new ASP.NET Web Application project called Chapter_14. Rename the default Web Page created by Visual Studio from WebPage1.aspx to SimpleTrace.aspx by selecting File, then Save WebForm1.aspx As.

2. Right-click anywhere on the page, and select Properties from the context menu. In the resulting dialog box (shown in the illustration on the following page), change the Page title to Simple Trace Example. Change Page Layout from GridLayout to FlowLayout. (If you use GridLayout, trace output and rendered page output can end up overlapping. Using FlowLayout prevents this.) Click OK.

   

3. Add a label to the page by dragging a label from the toolbox onto the form. In FlowLayout, the label will move to the upper left of the form no matter where you place it. Change the Text property to Trace is enabled for this page.

4. Click in an empty area of the page to select the DOCUMENT object, locate the trace property in the Properties window, and set its value to True.

5. Save SimpleTrace.aspx and the project, and then build the project.

6. Browse SimpleTrace.aspx using Microsoft Internet Explorer. The resulting screen should look similar to the following illustration:

   

The trace output includes such information as the contents of the cookies, forms, and querystring collections. Therefore, page-level tracing using the Trace attribute of the @ Page directive should be restricted to non-production systems because the output is visible to any client requesting that page. Although you can use one of the configuration options detailed in the next section to configure ASP.NET to provide page-level trace output only for requests from the local system, you’re best off sticking to application-level tracing on publicly available applications.

Enable application-level tracing in Web.config

1. In Visual Studio, open the Chapter_14 project created in the previous example.

2. Edit the Web.Config file by double-clicking on it in the Solution Explorer. Then, from the Edit menu, select Find And Replace, and then select Find. In the Find What text box, type <trace and then click Find Next. This will move your cursor to the beginning of the <trace> section. Change the <trace> element so that it looks like this:

   <trace     enabled="true    requestLimit="40    pageOutput="false    traceMode="SortByCategory    localOnly="true />
   

3. Save the changes to Web.config.

To allow access to the logged trace output, ASP.NET provides an HttpHandler that is mapped to a special URL called trace.axd. Appending trace.axd to the base URL for the application will display the list of currently available traces, as shown in the following illustration. Clicking View Details for a particular trace displays its output. Clicking Clear Current Trace clears all current entries from the trace log and allows more requests to be logged.

Determining If Tracing Is Enabled

To keep developers from wasting time attempting to write to the trace output when tracing is not enabled, the TraceContext class exposes a Boolean property called IsEnabled, which contains the current status of tracing for the page. This is especially helpful if you are performing expensive processing or database lookups as a part of your trace logging.

Using Trace.Write

The Trace.Write method of the TraceContext class is overloaded, and you can call it in one of the following three ways:

Trace.Write(string message) Trace.Write(string category, string message) Trace.Write(string category, string message, Exception errorInfo);

• message is a string containing the message to be written to the trace output.

• category is a string containing a description of the message (variable, statement, or whatever the developer wants as a category).

• errorInfo is an exception object, which allows exceptions to be logged to the trace output.

The category and errorInfo parameters allow developers to set up sophisticated application-wide tracing by defining specific categories of logged information and using them consistently across an application.

The following example shows how to create a page that tests if tracing is enabled for the page, and then writes a simple message to the trace output.

Write trace messages

1. In Visual Studio, open the Chapter_14 project used in the previous examples. Add a new page by right-clicking on the root of the project in the Solution Explorer, selecting Add, and then Add Web Form. Type the name of the form as TraceWrite.aspx.

2. Right-click anywhere on the page, and select Properties from the context menu. In the resulting dialog box, change Page Layout from GridLayout to FlowLayout, and then click OK.

3. Add a label to the page.

4. Change to HTML mode by clicking the HTML button on the bottom of the design window. Add Trace=“true” to the @ Page directive:

   <%@ Page Language="c#" AutoEventWireup="false"      Codebehind="TraceWrite.aspx.cs     Inherits="Chapter_14.TraceWrite" Trace="true" %>

5. Change to the code view by pressing F7. Scroll down to the Page_Load event handler and add the following code:

   if (Trace.IsEnabled  Label1.Text = "Writing 'Hello from Page!' to Trace Log..." Trace.Write("Hello from Page!") }

6. Save TraceWrite.aspx, its code-behind file, and the project, and then build the project.

7. Browse TraceWrite.aspx using Internet Explorer. The resulting screen should look similar to the illustration on the nexy page. (Notice the hello message on the first line under the Trace Information heading.)

   

Using Trace.Warn

Another method offered by the TraceContext class for writing to the trace output is Trace.Warn. The primary difference between the two methods is that output written with Trace.Warn appears in the trace output as red text, making it ideal for highlighting important entries such as exceptions.