TraceSource

In my post How to control whether to output Trace, I discussed how to output behavior of Trace object. We can control destination with the system.diagnostics.trace section, and control the when to output Trace by using listener's filter or using switch.

But .net offer a better object to replace Trace object, that is TraceSource. It explicitly explicitly works with listener and switch to give user a better control. Here is an article on this issue. Below is some demo code. For new application, we should only use TraceSource object instead of Trace. Please note that now you can use both Listner filter and switch to control whether to output. The reason to use TraceSource over Trace is that TraceSource is instance object, Trace is a static object. You may use different TraceSource to trace different component. This is especially useful to third party component so that their trace may be independent of user's trace. Here is another artical Extending System.Diagnostics

class Program { public static TraceSource MasterTraceSource = new TraceSource("TraceSourceApp"); static void Main(string[] args) { Trace.WriteLine("program started..."); Trace.Assert(1 != 1, "something wrong"); // MasterTraceSource.TraceInformation("Trace information"); MasterTraceSource.TraceEvent(TraceEventType.Error, 1, "Error message."); MasterTraceSource.TraceEvent(TraceEventType.Verbose, 2, "Warning message."); MasterTraceSource.Close(); return; } } <sources> <source name="TraceSourceApp" switchName="sourceSwitch" switchType="System.Diagnostics.SourceSwitch"> <listeners> <add name="consoleListner" type="System.Diagnostics.ConsoleTraceListener"> <filter type="System.Diagnostics.EventTypeFilter" initializeData="Error"/> </add> <remove name="Default"/> </listeners> </source> </sources> <switches> <add name="sourceSwitch" value="Verbose"/> </switches>

How to contro whether output Trace

We already know Listner affect Trace object's output destination. But that is only an aspect of behavior, another aspect is whether the message should be output. The control this we can test a configurable switch's value to determine whether we should output the message. But we need to manually Test the the switch, or we need to write a helper class to wrap the test code. Here is the code.

class Program { static TraceSwitch switch1 = new TraceSwitch("global", "global switch"); static void Main(string[] args) { Trace.WriteLine(switch1.Level); Trace.Assert(2 == 1, "error message is here, because assert fail(condition is false)"); if (switch1.TraceError) { Trace.TraceError("An error happend"); } if (switch1.TraceInfo) { Trace.TraceInformation("just for your information"); } if (switch1.TraceWarning) { Trace.TraceWarning("just a warning"); } } }

We can also use another filter feature of Listener, without using switch1.

<sharedListeners> <add type="System.Diagnostics.ConsoleTraceListener" name="Console"> <filter type="System.Diagnostics.EventTypeFilter" initializeData="Error"/> </add> </sharedListeners>

Trace.Assert

In nunit or other test framework, we have Assert object to test whether result is as expected. If assert is false, then en exception is through. Trace.Assert is provide similar function, but for different purpose. It is for diagnostic. When assert is false, it output the error message to a listener. There is default listener in .net, which is DefaultTraceListener An instance of this class is automatically added to the Debug..::.Listeners and Trace..::.Listeners collections. Explicitly adding a second DefaultTraceListener causes duplicate messages in the debugger output window and duplicate message boxes for asserts. The DefaultTraceListner looks like throwing an exception, but it is not actually an exception, it is the output behavior. If you remove the DefaultTraceListner, you will not see this output.