For debugging purposes (or program logging in the release version) it is useful to log messages to a trace listener. The API framework therefore provides a TraceListener class. 

In your API program, you just add your own trace listener to the System.Diagnostics.Trace.Listeners: 

private Eplan.EplApi.Base.TraceListener m_oTrace;
//.
//.
//.
    m_oTrace= new Eplan.EplApi.Base.TraceListener();
//.
//.
//.
public bool Execute(ActionCallingContext ctx )
{
    System.Diagnostics.Trace.Listeners.Add(m_oTrace);
    System.Diagnostics.Trace.WriteLine(" Begin Execute ");
//.
//.
}

Dim m_oTrace As Eplan.EplApi.Base.TraceListener
'...
   m_oTrace= New Eplan.EplApi.Base.TraceListener()
Public Function Execute(ByVal ctx as ActionCallingContext)as Boolean Implements IEplAction.Execute
    System.Diagnostics.Trace.Listeners.Add(m_oTrace)
    System.Diagnostics.Trace.WriteLine(" Begin Execute ")
'...
'...

[C#]

By this all further trace outputs are visible in the windows trace management and -- as the case may be -- written to the EPLAN log database at the program end. 

TRACE: .\Actions\AfCommandLineInterpreter.cpp(18) : AfCommandLineInterpreter::execute: CSharpAction
TRACE: .\Actions\AfAction.cpp(123) : Execute Action: URCheckRightsForAction
TRACE: .\Actions\AfAction.cpp(123) : Execute Action: CSharpAction
TRACE: u:\eplanw3_1.0_vc7.1\eplan\extensions\api_demosfue\v_1.0\eplan.w3addin.demo1\csharpaction.cs(24) : Begin Execute