Log duration of an ASP Web API action

Something like this should do the trick...

public class StopwatchAttribute : ActionFilterAttribute
{
    private const string StopwatchKey = "StopwatchFilter.Value";

    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        base.OnActionExecuting(actionContext);

        actionContext.Request.Properties[StopwatchKey] = Stopwatch.StartNew();
    }

    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        base.OnActionExecuted(actionExecutedContext);

        Stopwatch stopwatch = (Stopwatch)actionExecutedContext.Request.Properties[StopwatchKey];
        // TODO something useful with stopwatch.Elapsed
        Trace.WriteLine("Elapsed = " + stopwatch.Elapsed);
    }
}

Here we store a new Stopwatch in the request properties and stop it when the request has completed.