<add> Element for <listeners> for <source>

Adds a listener to the Listeners collection for a trace source.



<add name="name"   
  type="TraceListenerClassName, Version, Culture, PublicKeyToken"  

Attributes and Elements

The following sections describe attributes, child elements, and parent elements.


Attribute Description
type Required attribute.

Specifies the type of the listener. You must use a string that meets the requirements specified in Specifying Fully Qualified Type Names.
initializeData Optional attribute.

The string passed to the constructor for the specified class. A ConfigurationException is thrown if the class does not have a constructor that takes a string.
name Optional attribute.

Specifies the name of the listener.
traceOutputOptions Optional attribute.

Specifies the TraceOutputOptions property value for the trace listener.
[custom attributes] Optional attributes.

Specifies the value for listener-specific attributes identified by the GetSupportedAttributes method for that listener. Delimiter is an example of an extra attribute unique to the DelimitedListTraceListener class.

Child Elements

Element Description
<filter> Adds a filter to a listener in the Listeners collection for a trace source.

Parent Elements

Element Description
configuration The root element in every configuration file used by the common language runtime and .NET Framework applications.
system.diagnostics Specifies trace listeners that collect, store, and route messages and the level where a trace switch is set.
sources Contains trace sources that initiate tracing messages.
source Specifies a trace source that initiates tracing messages.
listeners Specifies listeners that collect, store, and route messages.


The listener classes shipped with the .NET Framework derive from the TraceListener class.

If you do not specify the name attribute of the trace listener, the Name property of the trace listener defaults to an empty string (""). If your application has only one listener, you can add it without specifying a name, and you can remove it by specifying an empty string for the name. However, if your application has more than one listener, you should specify a unique name for each trace listener, which allows you to identify and manage individual trace listeners in the TraceSource.Listeners collection.


Adding more than one trace listener of the same type and with the same name results in only one trace listener of that type and name being added to the Listeners collection. However, you can programmatically add multiple identical listeners to the Listeners collection.

The value for the initializeData attribute depends on the type of listener you create. Not all trace listeners require that you specify initializeData.


When you use the initializeData attribute, you may get the compiler warning "The 'initializeData' attribute is not declared." This warning occurs because the configuration settings are validated against the abstract base class TraceListener, which does not recognize the initializeData attribute. Typically, you can ignore this warning for trace listener implementations that have a constructor that takes a parameter.

The following table shows the trace listeners that are included with the .NET Framework and describes the value of their initializeData attributes.

Trace listener class initializeData attribute value
System.Diagnostics.ConsoleTraceListener The useErrorStream value for the ConsoleTraceListener constructor. Set the initializeData attribute to "true" to write trace and debug output to the standard error stream; set it to "false" to write to the standard output stream.
System.Diagnostics.DelimitedListTraceListener The name of the file the DelimitedListTraceListener writes to.
System.Diagnostics.EventLogTraceListener The name of an existing event log source.
System.Diagnostics.EventSchemaTraceListener The name of the file that the EventSchemaTraceListener writes to.
System.Diagnostics.TextWriterTraceListener The name of the file that the TextWriterTraceListener writes to.
System.Diagnostics.XmlWriterTraceListener The name of the file that the XmlWriterTraceListener writes to.

Configuration File

This element can be used in the machine configuration file (Machine.config) and the application configuration file.


The following example shows how to use <add> elements to add the listeners console and textListener to the Listeners collection for the trace source TraceSourceApp. The textListener listener writes trace output to the file myListener.log.

      <source name="TraceSourceApp" switchName="sourceSwitch"   
          <add name="console"   
          <add name="textListener"/>  
          <remove name="Default"/>  
      <add name="textListener"   
      <add name="sourceSwitch" value="Warning"/>  

See Also

Trace and Debug Settings Schema
Trace Listeners