This is what's killing you:
task.Wait();
That's blocking the UI thread until the task has completed - but the task is an async method which is going to try to get back to the UI thread after it "pauses" and awaits an async result. It can't do that, because you're blocking the UI thread...
There's nothing in your code which really looks like it needs to be on the UI thread anyway, but assuming you really do want it there, you should use:
private async void Button_Click(object sender, RoutedEventArgs
{
Task<List<MyObject>> task = GetResponse<MyObject>("my url");
var items = await task;
// Presumably use items here
}
Or just:
private async void Button_Click(object sender, RoutedEventArgs
{
var items = await GetResponse<MyObject>("my url");
// Presumably use items here
}
Now instead of blocking until the task has completed, the Button_Click method will return after scheduling a continuation to fire when the task has completed. (That's how async/await works, basically.)
Note that I would also rename GetResponse to GetResponseAsync for clarity.