Custom Web Api asynchronous filter implementation
Here is couple of things that I could done;


Implement a logger class, Logging with "Trace.WriteLine" is not usefull for everycase.
Override allow multiple; public override bool AllowMultiple => false;
Log the error...

public sealed class LogActionFilter : FilterAttribute, IActionFilter
{
    private readonly ILogManager _logManager;

    public LogActionFilter(ILogManager logManager)
    {
        _logManager = logManager;
    }

    private readonly Stopwatch _stopwatch = new Stopwatch();

    public override bool AllowMultiple =&gt; false;

    public async Task&lt;HttpResponseMessage&gt; ExecuteActionFilterAsync(HttpActionContext actionContext,
                                                                    CancellationToken cancellationToken,
                                                                    Func&lt;Task&lt;HttpResponseMessage&gt;&gt; continuation)
    {
        await InternalActionExecuting(actionContext);

        if (actionContext.Response != null)
        {
            return actionContext.Response;
        }

        HttpActionExecutedContext executedContext;

        try
        {
            var response = await continuation();

            executedContext = new HttpActionExecutedContext(actionContext, null)
                              {
                                  Response = response
                              };
        }
        catch (Exception exception)
        {
            executedContext = new HttpActionExecutedContext(actionContext, exception);
            _logManager.Error(exception, "LogAction recorded an error");
        }

        await InternalActionExecuted(executedContext);
        _stopwatch.Reset();

        return executedContext.Response;
    }

    private Task InternalActionExecuting(HttpActionContext actionContext)
    {
        _stopwatch.Start();
        return Log("Executing", actionContext, 0);
    }

    private Task InternalActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        _stopwatch.Stop();
        return Log("Executed", actionExecutedContext.ActionContext, _stopwatch.ElapsedMilliseconds);
    }

    private Task Log(string action, HttpActionContext actionContext, long elapsedTime)
    {
        return Task.Run(() =&gt;
                        {
                            var controllerName = actionContext.ControllerContext.ControllerDescriptor.ControllerName;
                            var actionName = actionContext.ActionDescriptor.ActionName;
                            var parameters = string.Join(", ", actionContext.ActionArguments.Values.Select(x =&gt; x).ToArray());

                            var message = $"{action}: ctrl: {controllerName}, act: {actionName}, params: {parameters}" +
                                          $"{(elapsedTime &gt; 0 ? "took (ms): " + elapsedTime : string.Empty)}";
#if (DEBUG)
                            Trace.WriteLine(message, "Action filter log");
#elif (RELEASE)
                            _logManager.Debug(null, message, "Action filter log");
#endif

                        });
    }
}