ASP.NET 4 in VB 2010 : Page Tracing (part 2) - Writing Trace Information

3. Writing Trace Information

The default trace log provides a set of important information that can allow you to monitor some important aspects of your application, such as the current state contents and the time taken to execute portions of code. In addition, you'll often want to generate your own tracing messages. For example, you might want to output the value of a variable at various points in execution so you can compare it with an expected value. Similarly, you might want to output messages when the code reaches certain points in execution so you can verify that various procedures are being used (and are used in the order you expect). Once again, these are tasks you can also achieve using Visual Studio debugging, but tracing is an invaluable technique when you're working with a web application that's been deployed to a web server.

To write a custom trace message, you use the Write() method or the Warn() method of the built-in Trace object. These methods are equivalent. The only difference is that Warn() displays the message in red lettering, which makes it easier to distinguish from other messages in the list. Here's a code snippet that writes a trace message when the user clicks a button:

Protected Sub cmdWrite_Click(ByVal sender As Object, _
  ByVal e As EventArgs) Handles cmdWrite.Click

    Trace.Write("About to place an item in session state.")
    Session("Test") = "Contents"
    Trace.Write("Placed item in session state.")
End Sub

These messages appear in the trace information section of the page, along with the default messages that ASP.NET generates automatically (see Figure 11).

Figure 11. Custom trace messages

You can also use an overloaded method of Write() or Warn() that allows you to specify the category. A common use of this field is to indicate the current method, as shown in Figure 12.

Figure 12. A categorized trace message

Protected Sub cmdWriteCategory_Click(ByVal sender As Object, _
  ByVal e As System.EventArgs) Handles cmdWriteCategory.Click

    Trace.Write("cmdWriteCategory_Click", _
      "About to place an item in session state.")
    Session("Test") = "Contents"
    Trace.Write("cmdWriteCategory_Click", _
      "Placed item in session state.")
End Sub

Alternatively, you can supply category and message information with an exception object that will automatically be described in the trace log, as shown in Figure 13.

Figure 13. An exception trace message

Protected Sub cmdError_Click(ByVal sender As Object, _
  ByVal e As EventArgs) Handles cmdError.Click

    Try
        DivideNumbers(5, 0)
    Catch err As Exception
        Trace.Warn("cmdError_Click", "Caught Error", err)
    End Try
End Sub

Private Function DivideNumbers(ByVal number As Decimal, _
  ByVal divisor As Decimal) As Decimal
    Return number/divisor
End Sub

By default, trace messages are listed in the order they were written by your code. Alternatively, you can specify that messages should be sorted by category using the TraceMode attribute in the Page directive:

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

or the TraceMode property of the Trace object in your code:

Trace.TraceMode = TraceMode.SortByCategory