How does System.Diagnostics.Tracing.EventSource.IsEnabled work?

Question

When using a custom event source e.g.:

[EventSource(Name = "MyEventSource")]
public partial class CustomEventSource : EventSource
{
}

There is an IsEnabled method on the EventSource class:

EventSource.IsEnabled(eventLevel, eventKeywords)

https://msdn.microsoft.com/en-us/library/hh393402(v=vs.110).aspx

How does this method determine whether the event is 'Enabled' for the level and keywords? There doesn't seem to be any solid documentation on this. On my implementation the method is returning false and I am not sure what needs to be done in order to make it return true.

It takes two to do this tango, the event source and the event listener. The source doesn't do the work until a somebody signals interest in the events. If the listener is a .net program then you'd use [EventListener.EnableEvents()](https://msdn.microsoft.com/en-us/library/hh393378(v=vs.110).aspx). Or you'd for example use the Windows logman.exe utility to [create a data collector](https://blogs.technet.microsoft.com/askperf/2008/05/13/two-minute-drill-logman-exe), try that first. — Hans Passant, Jul 31 '18 at 13:15

Freggar · Accepted Answer · 2018-07-31T11:27:18.027

Seems like you need to attach an EventListener to your EventSource to enable it:

class CustomEventListener : EventListener
{
    protected override void OnEventWritten(EventWrittenEventArgs eventData)
    {
    }
}

void Main() 
{
    var src = new CustomEventSource();
    var listener = new CustomEventListener();
    Console.WriteLine(src.IsEnabled(EventLevel.LogAlways, EventKeywords.None)); // false
    listener.EnableEvents(src, EventLevel.Error);

    Console.WriteLine(src.IsEnabled(EventLevel.LogAlways, EventKeywords.None)); // true
    Console.WriteLine(src.IsEnabled(EventLevel.Critical, EventKeywords.None)); // true
    Console.WriteLine(src.IsEnabled(EventLevel.Verbose, EventKeywords.None)); // false
}

EDIT:

I also found the EvenSource.SendCommand method which can take EventCommand.Enable as an argument but that only throws an ArgumentException for me. Yes, the documentation for EventSource is really bad.

Ben Hall · Answer 2 · 2018-07-31T13:20:56.560

From the source code for public bool IsEnabled(EventLevel level, EventKeywords keywords):

Returns true if events with greater than or equal 'level' and have one of 'keywords' set are enabled.

Note that the result of this function is only an approximation on whether a particular event is active or not. It is only meant to be used as way of avoiding expensive computation for logging when logging is not on, therefore it sometimes returns false positives (but is always accurate when returning false). EventSources are free to have additional filtering.

Note that false return is accurate so you need to look at your level and keywords.

@Hans is correct. I have neglected to point out that you need to start collecting the events for them to be enabled. You can do this programmatically or make use of a range of tools such as PerfView.

How does System.Diagnostics.Tracing.EventSource.IsEnabled work?

2 Answers2