c# - xUnit Async Test in Callback -

this current test. succeeds, should obvious not do. how make test fail?

[fact] public async task executecallback() {     timer timer = new timer();     timer.elapsed += (sender, e) =>     {         new myclassundertest((param) =>         {             assert.null(param);             assert.notnull(param);             timer.stop();         });     };     timer.interval = 500;     timer.start();     system.threading.thread.sleep(1000); } 

i want test inner logic of class (how executes provided action , parameters), , it's behavior when runs in eventhandler since tricky things threads.

as mentioned timer event handler runs on different thread nunit test. thrown exception not caught nunit, test succeeds.