[XML]

<?xml version="1.0"encoding="utf-8"?>
<configuration>
    <system.diagnostics>
        <switches>
            <addname="XPO"value="3"/>
        </switches>
    </system.diagnostics>
</configuration>

[XML]

<?xml version="1.0"encoding="utf-8"?>
<configuration>
    <system.diagnostics>
        <traceautoflush="true"indentsize="4">
            <listeners>
                <addname="LogFileTraceListener"type="System.Diagnostics.TextWriterTraceListener"
                    initializeData="trace.log"/>
                <removename="Default"/>
            </listeners>
        </trace>
        <switches>
            <addname="XPO"value="3"/>
        </switches>
    </system.diagnostics>
</configuration>

[C#]

System.Diagnostics.Trace.Listeners.Add(newMyTraceListner(textBox1));
...
class MyTraceListner:System.Diagnostics.TraceListener{
    TextBox outputWindow;
    public MyTraceListner(TextBoxoutputWindow){
        this.outputWindow=outputWindow;
    }
    public overridevoidWrite(stringmessage){
        outputWindow.AppendText(message);
    }
    public overridevoidWriteLine(stringmessage){
        outputWindow.AppendText(message+"\r\n");
    }
}