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EventHandler ev = null;
ev = (o, e) => {
  source.Event -= eh;
  DoSomething();
};
source.Event += eh;

await source.Event.WaitForEvent()
DoSomething();

source.Event += (o, e) => {
  source.Event -= eh;
  DoSomething();
};

oArgType o; 
hArgType h;
await source.Event.WaitForEvent(out o, out h);
DoSomething(o, h);

public static Task<GpsLocationArgs> WaitForLocationChangedAsync(this IGps gps)
{
    var res = new TaskCompletionSource<GpsLocationArgs>();
    EventHandler<GpsLocation> eh = null;
    eh = (o, e) =>
    {
        gps.LocationChanged -= eh;
        res.TrySetResult(e);
    };
    gps.LocationChanged += eh;
    return res.Task;
}
// ....
var loc = await myGps.WaitForLocationChangedAsync();
Console.WriteLine($"({loc.Latitude}, {loc.Longitude})");

await source.WaitForEvent<EventArgs>(nameof(source.Event));

public static class EventExtensions
    {
        public static Task<T> WaitForEvent<T>(this object eventSource, string eventName) where T : EventArgs
        {
            var tcs = new System.Threading.Tasks.TaskCompletionSource<T>();
            var eventInfo = eventSource.GetType().GetEvent(eventName);

            EventHandler<T> eventHandler = null;
            eventHandler = new EventHandler<T>(
                (source, e) =>
                {
                    eventInfo.RemoveEventHandler(eventSource, eventHandler);
                    tcs.TrySetResult(e);
                }
            );
            eventInfo.AddEventHandler(eventSource, eventHandler);

            return tcs.Task;
        }
    }