为什么引发 PropertyChanged 事件会导致其他控件更新

Question

我有一个视图模型，它具有多个属性，这些属性数据绑定到多个控件。

当我在其中一个上引发 PropertyChanged 时，控件出乎意料地全部更新。我希望只有我提出事件的那个会更新。

对于我的表单，我有这个：

    public partial class MainForm : Form
    {
        AmountCalculatorVM amountCalculatorVM;
        public MainForm()
        {
            InitializeComponent();
        }

        private void setBindings()
        {
            textBoxTotalAmount.DataBindings.Add("Text", amountCalculatorVM, "TotalAmount");
            textBoxAverage.DataBindings.Add("Text", amountCalculatorVM, "Average",true, DataSourceUpdateMode.Never,null, "#.00");
            textBoxCount.DataBindings.Add("Text", amountCalculatorVM, "Count");
            listBoxLineAmounts.DataSource = amountCalculatorVM.Amounts;
        }

        private void MainForm_Load(object sender, EventArgs e)
        {
            amountCalculatorVM = new AmountCalculatorVM();
            setBindings();
        }

        private void buttonAddAmount_Click(object sender, EventArgs e)
        {
            if (int.TryParse(textBoxLineAmount.Text.Replace(",", ""), out int amount))
            {
                amountCalculatorVM.Amounts.Add(amount);
                textBoxLineAmount.Text = "";
                textBoxLineAmount.Focus();
            }
        }


        private void buttonClear_Click(object sender, EventArgs e)
        {
            textBoxLineAmount.Text = "";
            amountCalculatorVM.Amounts.Clear();
            textBoxLineAmount.Focus();
        }

    }

然后，对于我的视图模型，我有这个：

    class AmountCalculatorVM : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private readonly AmountList amounts;
        public BindingSource Amounts { get; }
        public int TotalAmount => amounts.TotalAmount;
        public int Count => amounts.Count;
        public decimal Average => amounts.Average;
        public AmountCalculatorVM()
        {
            amounts = new AmountList();
            Amounts = new BindingSource();
            Amounts.DataSource = amounts;
            Amounts.ListChanged += Amounts_ListChanged;
        }

        private void Amounts_ListChanged(object sender, ListChangedEventArgs e)
        {

            //Any one of these will cause all three textboxes to update in the form
            //I would think that with Count and Average commented out, the Count and 
            //Average textboxes would not update.
            OnPropertyChanged("TotalAmount"); 
            //OnPropertyChanged("Count"); 
            //OnPropertyChanged("Average");

            //Using any other word will not
            //OnPropertyChanged("SomeOtherRandomWord");

        }
    }

这里是 AmountList 类供参考：

 class AmountList : List<int>
    {
        public int TotalAmount
        {
            get
            {
                int total = 0;
                foreach (int amount in this)
                {
                    total += amount;
                }
                return total;
            }
        }

现在，出乎意料的是，如果将一个项目添加到金额列表中，所有三个文本框都会更新，这会触发 ListChanged，然后依次触发 PropertyChanged 事件。

我触发 PropertyChanged 的​​三个属性中的哪一个无关紧要，但如果我使用不同的值，它将不起作用 - 它需要是 TotalAmount、Count 或 Average。

我无法理解这种行为。我预计只有绑定到 TotalAmount 的文本框会被更新，而不是其他两个，因为似乎没有通知他们发生了更新。

有什么想法吗？

Answer 1

你为什么不这样实现 propertychanged：

public class Data : INotifyPropertyChanged
{
    // boiler-plate
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    // props
    private string name;
    public string Name
    {
        get { return name; }
        set { SetField(ref name, value, "Name"); }
    }
}

您现在可以在 setter 中控制触发事件的属性：

private string name;
public string Name
{
    get { return name; }
    set { SetField(ref name, value, "Name"); }
}

你知道我的意思吗？