/// <summary>
        /// コンストラクタ。
        /// </summary>
        /// <param name="command">非同期コマンド。</param>
        private AsyncCommandHolderBase(AsyncReactiveCommand <TParameter> command)
        {
            if (command == null)
            {
                return;
            }

            // 戻り値の通知先
            // 同値でも通知されるように DistinctUntilChanged を外す
            var result =
                new ReactiveProperty <TResult>(
                    default(TResult),
                    ReactivePropertyMode.RaiseLatestValueOnSubscribe)
                .AddTo(this.CompositeDisposable);

            var busy = new ReactiveProperty <bool>(false).AddTo(this.CompositeDisposable);

            command
            .Subscribe(
                async p =>
            {
                busy.Value = true;
                try
                {
                    result.Value = await this.Execute(this.ConvertParameter(p));
                }
                finally
                {
                    busy.Value = false;
                }
            })
            .AddTo(this.CompositeDisposable);

            this.Command    = command;
            this.CanExecute =
                command
                .CanExecuteToReadOnlyReactiveProperty()
                .AddTo(this.CompositeDisposable);
            this.IsBusy = busy;
            this.Result = result;
        }