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TraceSource Class

Updated: August 2010

Provides a set of methods and properties that enable applications to trace the execution of code and associate trace messages with their source.

Namespace:  System.Diagnostics
Assembly:  System (in System.dll)

Public Class TraceSource
Dim instance As TraceSource

The TraceSource class is used by applications to produce traces that can be associated with the application. TraceSource provides tracing methods that allow you to easily trace events, trace data, and issue informational traces. Trace output from TraceSource can be controlled by configuration file settings. The configuration file is located in the folder with the application executable and has the name of the application with the .config file name extension added. For example, the name of the configuration file for TraceSourceSample.exe is TraceSourceSample.exe.config. The configuration file can be used to determine where the trace information is to be sent and what levels of activity are to be traced. The following example shows the contents of a sample application configuration file.

      <source name="TraceTest" switchName="SourceSwitch" 
        switchType="System.Diagnostics.SourceSwitch" >
          <add name="console" />
          <remove name ="Default" />
      <!-- You can set the level at which tracing is to occur -->
      <add name="SourceSwitch" value="Warning" />
        <!-- You can turn tracing off -->
        <!--add name="SourceSwitch" value="Off" -->
      <add name="console" 
    <trace autoflush="true" indentsize="4">
        <add name="console" />

The TraceSource class is identified by the name of a source, typically the name of the application. The trace messages coming from a particular component can be initiated by a particular trace source, allowing all messages coming from that component to be easily identified.

TraceSource defines tracing methods but does not actually provide any specific mechanism for generating and storing tracing data. The tracing data is produced by trace listeners, which are plug-ins that can be loaded by trace sources.


You should not call the tracing methods during finalization. Doing so can result in an ObjectDisposedException being thrown.

You can customize the tracing output's target by adding or removing TraceListener instances to or from the collection stored in the TraceSource.Listeners property. By default, trace output is produced using an instance of the DefaultTraceListener class. The preceding configuration file example demonstrates removing the DefaultTraceListener and adding a ConsoleTraceListener to produce the trace output for the trace source. For more information, see <listeners> Element for <source> and <sharedListeners> Element.


Adding a trace listener to the Listeners collection can cause an exception to be thrown while tracing, if a resource used by the trace listener is not available. The conditions and the exception thrown depend on the trace listener and cannot be enumerated in this topic. It may be useful to place calls to the TraceSource methods in try/catch blocks to detect and handle any exceptions from trace listeners.

The SourceSwitch class provides the means to dynamically control the tracing output. The preceding configuration file example shows how you can turn off tracing from a trace source and control the level at which tracing occurs. You can modify the value of the source switch without recompiling your application. For information on using the configuration file to set a switch, see Switch and How to: Configure Trace Switches.


If you modify a configuration file while an application is executing, the application must be stopped and restarted or the Refresh method must be called before the new settings take effect.

The TraceEventType enumeration is used to define the event type of the trace message. Trace filters use the TraceEventType to determine if a trace listener should produce the trace message.

The trace listeners can optionally have an additional layer of filtering through a trace filter. If a trace listener has an associated filter, the listener calls the ShouldTrace method on that filter to determine whether or not to produce the trace information.

The trace listeners use the values of the Trace class properties Indent, IndentSize, and AutoFlush to format trace output. You can use configuration file attributes to set the Indent, IndentSize, and AutoFlush properties. The following example sets the AutoFlush property to false and the IndentSize property to 3.

    <trace autoflush="false" indentsize="3" />

The following code example shows the use of the TraceSource class to forward traces to listeners. The example also demonstrates switch and filter usage.

' The following configuration file can be used with this sample. 
' When using a configuration file #define ConfigFile. 
'    <system.diagnostics> 
'        <sources> 
'            <source name="TraceTest" switchName="SourceSwitch" switchType="System.Diagnostics.SourceSwitch" > 
'                <listeners> 
'                    <add name="console" type="System.Diagnostics.ConsoleTraceListener" initializeData="false" /> 
'                    <remove name ="Default" /> 
'                </listeners> 
'            </source> 
'        </sources> 
'        <switches> 
'            <!-- You can set the level at which tracing is to occur --> 
'            <add name="SourceSwitch" value="Warning" /> 
'            <!-- You can turn tracing off --> 
'            <!--add name="SourceSwitch" value="Off" --> 
'        </switches> 
'        <trace autoflush="true" indentsize="4"></trace> 
'    </system.diagnostics> 
#Const TRACE = True 
'#Const ConfigFile = True 

Imports System
Imports System.Collections
Imports System.Diagnostics
Imports System.Reflection
Imports System.IO
Imports System.Security.Permissions

Class TraceTest
    ' Initialize the trace source. 
    Private Shared ts As New TraceSource("TraceTest")
    <SwitchAttribute("SourceSwitch", GetType(SourceSwitch))> _
    Shared Sub Main()
            ' Initialize trace switches.
#If (ConfigFile = False) Then 
            Dim sourceSwitch As New SourceSwitch("SourceSwitch", "Verbose")
            ts.Switch = sourceSwitch
            Dim idxConsole As New Integer()
            idxConsole = ts.Listeners.Add(New System.Diagnostics.ConsoleTraceListener())
            ts.Listeners(idxConsole).Name = "console"
#End If
            ts.Listeners("console").TraceOutputOptions = ts.Listeners("console").TraceOutputOptions Or TraceOptions.Callstack
            ts.TraceEvent(TraceEventType.Warning, 1)
            ts.Listeners("console").TraceOutputOptions = TraceOptions.DateTime
            ' Issue file not found message as a warning.
            ts.TraceEvent(TraceEventType.Warning, 2, "File Test not found")
            ' Issue file not found message as a verbose event using a formatted string.
            ts.TraceEvent(TraceEventType.Verbose, 3, "File {0} not found.", "test")
            ' Issue file not found message as information.
            ts.TraceInformation("File {0} not found.", "test")
            ts.Listeners("console").TraceOutputOptions = ts.Listeners("console").TraceOutputOptions Or TraceOptions.LogicalOperationStack
            ' Issue file not found message as an error event.
            ts.TraceEvent(TraceEventType.Error, 4, "File {0} not found.", "test")
            ' Test the filter on the ConsoleTraceListener.
            ts.Listeners("console").Filter = New SourceFilter("No match")
            ts.TraceData(TraceEventType.Error, 5, "SourceFilter should reject this message for the console trace listener.")
            ts.Listeners("console").Filter = New SourceFilter("TraceTest")
            ts.TraceData(TraceEventType.Error, 6, "SourceFilter should let this message through on the console trace listener.")
            ts.Listeners("console").Filter = Nothing 
            ' Use the TraceData method. 
            ts.TraceData(TraceEventType.Warning, 7, New Object())
            ts.TraceData(TraceEventType.Warning, 8, New Object() {"Message 1", "Message 2"})
            ' Activity tests.
            ts.TraceEvent(TraceEventType.Start, 9, "Will not appear until the switch is changed.")
            ts.Switch.Level = SourceLevels.ActivityTracing Or SourceLevels.Critical
            ts.TraceEvent(TraceEventType.Suspend, 10, "Switch includes ActivityTracing, this should appear")
            ts.TraceEvent(TraceEventType.Critical, 11, "Switch includes Critical, this should appear")
            Console.WriteLine("Press any key to exit.")
        Catch e As Exception
            ' Catch any unexpected exception.
            Console.WriteLine("Unexpected exception: " + e.ToString())
        End Try 

    End Sub 'Main

    Public Shared Sub DisplayProperties(ByVal ts As TraceSource)
        Console.WriteLine("TraceSource name = " + ts.Name)
        Console.WriteLine("TraceSource switch level = " + ts.Switch.Level.ToString())
        Console.WriteLine("TraceSource switch = " + ts.Switch.DisplayName.ToString())
        Dim switches As SwitchAttribute() = SwitchAttribute.GetAll([Assembly].GetExecutingAssembly())
        Dim i As Integer 
        For i = 0 To switches.Length - 1
            Console.WriteLine("Switch name = " + switches(i).SwitchName.ToString())
            Console.WriteLine("Switch type = " + switches(i).SwitchType.ToString())
        Next i

#If (ConfigFile) Then 
        ' Get the custom attributes for the TraceSource.
        Console.WriteLine("Number of custom trace source attributes = " + ts.Attributes.Count)
        Dim de As DictionaryEntry
        For Each de In ts.Attributes
            Console.WriteLine("Custom trace source attribute = " + de.Key + "  " + de.Value)
        Next de
        ' Get the custom attributes for the trace source switch. 
        For Each de In ts.Switch.Attributes
            Console.WriteLine("Custom switch attribute = " + de.Key + "  " + de.Value)
        Next de
#End If
        Console.WriteLine("Number of listeners = " + ts.Listeners.Count.ToString())
        Dim traceListener As TraceListener
        For Each traceListener In ts.Listeners
            Console.Write("TraceListener: " + traceListener.Name + vbTab)
            ' The following output can be used to update the configuration file.
            Console.WriteLine("AssemblyQualifiedName = " + traceListener.GetType().AssemblyQualifiedName)
        Next traceListener

    End Sub 'DisplayProperties
End Class 'TraceTest 


This type is thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0




August 2010

Corrected type safety information.

Content bug fix.

August 2008

Added note about exceptions thrown by trace listeners.

Customer feedback.

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