C#XamarinForms-呼叫事件總是回傳null

大家好，很抱歉我的英語不好。

我正在 Xamarin Forms 上開發應用程式，但遇到了event invocation.

我有一個帶有各種標簽的 ContentView 和兩個按鈕來編輯和洗掉有問題的物件。如您所見，我嘗試了各種事件呼叫方法，但是在EventHandler呼叫它時始終為 null。

InstrumentCard.xaml.cs 代碼：

public partial class InstrumentCard : ContentView
    {
        public event EventHandler<InstrumentEventArgs> DeleteClicked;
        public event EventHandler<InstrumentEventArgs> EditClicked;

        private Instrument _i;

        public InstrumentCard(Instrument i)
        {
            InitializeComponent();

            _i = i;
            lblID.Text = i.ID.ToString();
            lblName.Text = i.Nome;
            lblQty.Text = $"Quantità: {i.Qty.ToString()}";
        }

        public Instrument GetInstrument()
        {
            return _i;
        }

        private void btnEdit_Clicked(object sender, EventArgs e)
        {
            RaiseEdit(new InstrumentEventArgs { Instrument = _i});
        }

        private void RaiseEdit(InstrumentEventArgs args)
        {
            EditClicked?.Invoke(this, args);
        }

        private void btnDelete_Clicked(object sender, EventArgs e)
        {
            DeleteClicked?.Invoke(this, new InstrumentEventArgs
            {
                Instrument = _i
            });
        }
    }

MainPage.xaml.cs 代碼：

    public partial class MainPage : ContentPage
    {
        List<Instrument> instruments = new List<Instrument>();
        public MainPage()
        {
            InitializeComponent();
        }

        protected override async void OnAppearing()
        {
            base.OnAppearing();
            
            Reload();
        }

        public async void Reload()
        { 
            List<Instrument> instruments = await App.Database.GetInstrumentsAsync();

            lblCounter.Text = $"Ci sono {instruments.Count} strumenti";

            if (instruments.Count == 0)
            {
                stcNoData.IsVisible = true;
                stcData.IsVisible = false;
                stcDock.IsVisible = false;
            }
            else
            {
                stcNoData.IsVisible = false;
                stcData.IsVisible = true;
                stcDock.IsVisible = true;

                stcInstruments.Children.Clear();

                instruments.ForEach(instr =>
                {
                    InstrumentCard ic = new InstrumentCard(instr);
                    ic.DeleteClicked  = Ic_DeleteClicked;
                    ic.EditClicked  = Ic_EditClicked;

                    stcInstruments.Children.Add(new InstrumentCard(instr));
                });
            }
        }

        private void Ic_EditClicked(object sender, InstrumentEventArgs e)
        {
            Navigation.PushAsync(new AddInstrument(e.Instrument));
        }

        private void Ic_DeleteClicked(object sender, InstrumentEventArgs e)
        {
            stcInstruments.Children.Remove(sender as InstrumentCard);
            App.Database.DeleteInstrumentAsync(e.Instrument);
            Reload();
        }
}

當我除錯該方法時...

我不知道出了什么問題。感謝您的幫助

uj5u.com熱心網友回復：

問題就在這里

instruments.ForEach(instr =>
{
  //here you create your instrument card
  InstrumentCard ic = new InstrumentCard(instr);
  ic.DeleteClicked  = Ic_DeleteClicked;
  ic.EditClicked  = Ic_EditClicked;
  //here you should use it but instead you create another one
  stcInstruments.Children.Add(new InstrumentCard(instr));
});

只需將您創建的實體添加到您訂閱事件的StackPanel（我希望它是StackPanel）

stcInstruments.Children.Add(ic);

標籤：C＃ 沙马林 xamarin.forms 事件处理

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