c# - Thread Blocking the UI -

i following example c# in nutshell. according text following code supposed non blocking, find form not display until 5 seconds have passed.

private void form1_load(object sender, eventargs e) {     var tcs = new taskcompletionsource<int>();      new thread(() => {thread.sleep(5000); tcs.setresult(42); }).start();      task<int> task = tcs.task;     messagebox.show(task.result.tostring()); } 

i have feeling it's thread.sleep() , instead of putting new thread sleep, it's putting main thread sleep.

why blocking ui thread?

when call task.result.tostring() (in messagebox.show) task class has mechanism waits task finished before giving result (as doesn't have until task finishes. here's proof:

private void form1_load(object sender, eventargs e) {     var tcs = new taskcompletionsource<int>();      new thread(() => {thread.sleep(5000); tcs.setresult(42); }).start();      task<int> task = tcs.task;     thread.sleep(2500);     messagebox.show("waited 2.5secs on ui thread.");     messagebox.show(task.result.tostring()); } 

you see shows 2.5sec message box before messagebox 42. (in fact, 2.5 seconds before that).

what looking this:

private void form1_load(object sender, eventargs e) {     var tcs = new taskcompletionsource<int>();      new thread(() => {thread.sleep(5000); tcs.setresult(42); }).start();      task<int> task = tcs.task;     task.continuewith(t => messagebox.show(t.result.tostring())); } 

this not freeze ui thread.