イベントをタスクに変換するために使用できる 6 つのメソッドの小さなツールボックスを次に示します。
/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on <see cref="EventHandler"/>, to a Task.</summary>
public static Task EventToAsync(
Action<EventHandler> addHandler,
Action<EventHandler> removeHandler)
{
var tcs = new TaskCompletionSource<object>();
addHandler(Handler);
return tcs.Task;
void Handler(object sender, EventArgs e)
{
removeHandler(Handler);
tcs.SetResult(null);
}
}
/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on <see cref="EventHandler{TEventArgs}"/>, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TEventArgs>(
Action<EventHandler<TEventArgs>> addHandler,
Action<EventHandler<TEventArgs>> removeHandler)
{
var tcs = new TaskCompletionSource<TEventArgs>();
addHandler(Handler);
return tcs.Task;
void Handler(object sender, TEventArgs e)
{
removeHandler(Handler);
tcs.SetResult(e);
}
}
/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on a supplied event delegate type, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TDelegate, TEventArgs>(
Action<TDelegate> addHandler, Action<TDelegate> removeHandler)
{
var tcs = new TaskCompletionSource<TEventArgs>();
TDelegate handler = default;
Action<object, TEventArgs> genericHandler = (sender, e) =>
{
removeHandler(handler);
tcs.SetResult(e);
};
handler = (TDelegate)(object)genericHandler.GetType().GetMethod("Invoke")
.CreateDelegate(typeof(TDelegate), genericHandler);
addHandler(handler);
return tcs.Task;
}
/// <summary>Converts a named .NET event, conforming to the standard .NET event
/// pattern based on <see cref="EventHandler"/>, to a Task.</summary>
public static Task EventToAsync(object target, string eventName)
{
var type = target.GetType();
var eventInfo = type.GetEvent(eventName);
if (eventInfo == null) throw new InvalidOperationException("Event not found.");
var tcs = new TaskCompletionSource<object>();
EventHandler handler = default;
handler = new EventHandler((sender, e) =>
{
eventInfo.RemoveEventHandler(target, handler);
tcs.SetResult(null);
});
eventInfo.AddEventHandler(target, handler);
return tcs.Task;
}
/// <summary>Converts a named .NET event, conforming to the standard .NET event
/// pattern based on <see cref="EventHandler{TEventArgs}"/>, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TEventArgs>(
object target, string eventName)
{
var type = target.GetType();
var eventInfo = type.GetEvent(eventName);
if (eventInfo == null) throw new InvalidOperationException("Event not found.");
var tcs = new TaskCompletionSource<TEventArgs>();
EventHandler<TEventArgs> handler = default;
handler = new EventHandler<TEventArgs>((sender, e) =>
{
eventInfo.RemoveEventHandler(target, handler);
tcs.SetResult(e);
});
eventInfo.AddEventHandler(target, handler);
return tcs.Task;
}
/// <summary>Converts a generic Action-based .NET event to a Task.</summary>
public static Task<TArgument> EventActionToAsync<TArgument>(
Action<Action<TArgument>> addHandler,
Action<Action<TArgument>> removeHandler)
{
var tcs = new TaskCompletionSource<TArgument>();
addHandler(Handler);
return tcs.Task;
void Handler(TArgument arg)
{
removeHandler(Handler);
tcs.SetResult(arg);
}
}
これらのメソッドはすべてTask、関連付けられたイベントの次の呼び出しで完了する を作成しています。このタスクは失敗したりキャンセルされたりすることはなく、正常に完了するだけです。
標準イベント（Progress<T>.ProgressChanged）での使用例：
var p = new Progress<int>();
//...
int result = await EventToAsync<int>(
h => p.ProgressChanged += h, h => p.ProgressChanged -= h);
// ...or...
int result = await EventToAsync<EventHandler<int>, int>(
h => p.ProgressChanged += h, h => p.ProgressChanged -= h);
// ...or...
int result = await EventToAsync<int>(p, "ProgressChanged");
非標準イベントでの使用例:
public static event Action<int> MyEvent;
//...
int result = await EventActionToAsync<int>(h => MyEvent += h, h => MyEvent -= h);
タスクが完了すると、イベントは登録解除されます。それより前に購読を解除するためのメカニズムは提供されていません。