Tracing | in .NET framework

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Tracing |
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Table 8-1. Commonly Used HTTP Status Codes Category Informational (100 199) Successful (200 299) Redirection (300 399) Number 100 101 200 204 301 305 307 Client Errors (400 499) 400 401 402 403 404 408 417 Server Error (500 599) 500 Description Continue Switching protocols OK No content Moved permanently Use proxy Temporary redirect Bad request Unauthorized Payment required Forbidden Not found Request timeout Expectation failed Internal Server Error
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The next section, labeled Trace Information, is the trace log (this section was shown briefly in 7 in the discussion of life cycle), which provides life-cycle information. This includes elapsed times, in seconds, since the page was initialized (the From First(s) column) and since the previous event in the life cycle (the From Last(s) column). You can add custom trace information to the trace log, as explained later in the next section. The next section in the trace, under the heading Control Tree, lists all the controls on the page in a hierarchical manner, including the name of the control, its type, and its size in bytes, on the page, in the ViewState state bag, and in control state. This is followed by Session and Application State summaries, and itemizations of the Cookies and Headers collections. Finally, there is a list of all the server variables.
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Inserting into the Trace Log
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You can add custom information to the trace output by writing to the Trace object. This object exposes two methods for putting your own statements into the trace log: Write and Warn. The only difference between the two methods is that Warn writes to the log in red. The Warn and Write methods can take either a single argument, two arguments, or two strings and an exception object (generated by the .NET Framework when using a try/catch block), as the following cases illustrate:
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Trace.Warn("Warning Message")
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Inserts a record into the trace log with the message passed in as a string.
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8: Errors, Exceptions, and Bugs, Oh My!
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Trace.Warn("Category","Warning Message")
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Inserts a record into the trace log with the category and message you pass in.
Trace.Warn("Category","Warning Message", excp)
Inserts a record into the trace log with a category, warning message, and exception. To see this in action, add the highlighted code from Example 8-3 to the code-behind file in your sample web site, DebuggingApp.
Protected Sub Page_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Load Trace.Write("In Page_Load") If Not IsPostBack Then Trace.Write("Page_Load", "Not PostBack.") ' Build 2 dimensional array for the lists ' First dimension contains bookname ' Second dimension contains ISBN number Dim books(,) As String = { _ {"Programming Silverlight", "0000000001"}, _ {"Programming .NET 3", "0000000002"}, _ {"Programming ASP.NET, 4th Edition", "0000000003"}, _ {"Programming Visual Basic 9", "0000000004"}, _ {"Programming C#, 5th Edition", "0000000005"}, _ {"Learning ASP.NET ", "0596513976"} _ } ' Now populate the list For i As Integer = 0 To books.GetLength(0) - 1 ' add both Text and Value ddlBooks.Items.Add(New ListItem(books(i, 0), books(i, 1))) Next End If End Sub Protected Sub ddlBooks_SelectedIndexChanged(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles ddlBooks.SelectedIndexChanged ' force an exception Try Dim a As Integer = 0 Dim b As Integer = 5 / a Catch ex As Exception Trace.Warn("User Action", "Calling b=5/a", ex) End Try ' check to verify that something has been selected If ddlBooks.SelectedIndex <> -1 Then lblDdl.Text = ddlBooks.SelectedItem.Text + " ---> ISBN: " _ + ddlBooks.SelectedItem.Value End If End Sub
Tracing |
The first message is added in the Page_Load method to signal that you ve entered that method:
Trace.Write("In Page_Load")
The second message is added if the page is not a postback:
If Not IsPostBack Then Trace.Write("Page_Load", "Not PostBack.")
This second message is categorized as Page_Load; using a category can help you organize the trace output. The effect of these two Write statements is shown in Figure 8-4.
The third message is added to demonstrate the process of inserting an exception into the error log. The ddlBooks_SelectedIndexChanged event handler now contains code to force an exception by dividing by zero. The code catches that exception and logs the exception with a Trace statement, as shown by the following code fragment:
Try Dim a As Integer = 0 Dim b As Integer = 5 / a Catch ex As Exception Trace.Warn("User Action", "Calling b=5/a", ex) End Try
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8: Errors, Exceptions, and Bugs, Oh My!
The output from this Trace statement is shown in Figure 8-5.
Because this Trace statement was written by calling the Warn method rather than the Write method, the trace output appears in red onscreen (though not in your printed copy of this book). Notice that string you passed in, Calling b=5/a, is displayed, followed by an error message extracted automatically from the exception object. Implementing Trace statements is easy, and when it s time to put your page into production, all these statements can remain in place. The only modification you need to make is to change the Trace attribute in the Page directive from true to false.
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