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Trace Logging to Page Output

Page-level tracing enables you to write debugging statements directly to a page's output, and conditionally run debugging code when tracing is enabled. To enable tracing for a page, include the following directive at the top of the page code:

<%@ Page Trace="true"%>

Trace statements can also be organized by category, using the TraceMode attribute of the Page directive. If no TraceMode attribute is defined, the default value is SortByTime.

<%@ Page Trace="true" TraceMode="SortByCategory" %>

The following example shows the default output when page-level tracing is enabled. Note that ASP.NET inserts timing information for important places in the page's execution lifecycle:

The page exposes a Trace property (of type TraceContext), which can be used to output debugging statements to the page output, provided tracing is enabled. Using TraceContext, you can write debugging statements using the Trace.Write and Trace.Warn methods, which each take a message string or a category and message string. Trace.Warn statements are identical to Trace.Write statements, except they are output in red.

// Trace(Message) Trace.Write("Beginning User Code..."); ... Trace.Warn("Array count is null!"); // Trace(Category, Message) Trace.Write("Custom Trace","Beginning User Code..."); ... Trace.Warn("Custom Trace","Array count is null!"); ' Trace(Message) Trace.Write("Beginning User Code...") ... Trace.Warn("Array count is Nothing!") ' Trace(Category, Message) Trace.Write("Custom Trace","Beginning User Code...") ... Trace.Warn("Custom Trace","Array count is null!")
When tracing is disabled (that is, when Trace="false" on the Page directive, or is not present), these statements do not run and no Trace output appears in the client browser. This makes it possible to keep debugging statements in production code and enable them conditionally at a later time.

Often you might need to run additional code to construct the statements to pass to the Trace.Write or Trace.Warn methods, where this code should only run if tracing is enabled for the page. To support this, Page exposes a Boolean property, Trace.IsEnabled, which returns true only if tracing is enabled for the page. You should check this property first to guarantee that your debugging code can only run when tracing is on.

if (Trace.IsEnabled) { for (int i=0; i<ds.Tables["Categories"].Rows.Count; i++) { Trace.Write("ProductCategory",ds.Tables["Categories"].Rows[i][0].ToString()); } } If Trace.IsEnabled Then For i=0 To ds.Tables("Categories").Rows.Count-1 Trace.Write("ProductCategory",ds.Tables("Categories").Rows(i)(0).ToString()) Next End if
The following example shows the use of Trace.Write and Trace.Warn to output debugging statements. Also note the use of the Trace.IsEnabled property to conditionally run extra debugging code. In this example, the trace information has been sorted by category.

Integration of ASP.NET Trace and System.Diagnostics.Trace New in 2.0

ASP.NET Trace messages can be now forwarded to System.Diagnostics.Trace in order to make them available to generic processing infrastructure and tools consuming System.Diagnostics.Trace messages. This feature can be enabled in the configuration section by setting writeToDiagnosticsTrace to true.

Likewise, System.Diagnostics.Trace messages can be consumed by ASP.NET Trace and displayed with the page trace messages in the page trace output or the application trace viewer. This is incredibly useful to trace the execution of generic .NET components within ASP.NET pages. This is accomplished by wiring up the WebPageTraceListener in the element of the configuration section, as shown in the sample below:

Note that trace option must be enabled during compilation of code containing System.Diagnostics.Trace statements for them to generate messages.

ASP.NET also provides a way to enable tracing for the entire application, not just a single page. For more about application-level tracing, click here.