Question

我在业务层中有一些逻辑根据输入限制了ComboBox选项，因此我需要更改底层BindingList中的值。但是当列表发生变化时，双向绑定将成为从UI到实体的单向绑定。

_mComboBox.DataBindings.Add("SelectedValue", _mEntity, "WifeCount");

分配按钮单击处理程序中存在问题的完整代码：

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace EnumDataBinding
{
    public partial class Form1 : Form
    {
        ComboBox _mComboBox = new ComboBox();
        Button _mCheckButton = new Button();
        Button _mAssignButton = new Button();

        BindingList<OptionValue> _mBindingList = new BindingList<OptionValue>();
        List<OptionValue> _mCacheList = new List<OptionValue>();

        Entity _mEntity = new Entity();

        public Form1()
        {
            InitializeComponent();

            // create a reset button
            _mCheckButton.Size = new Size(100, 30);
            _mCheckButton.Text = "Check";
            _mCheckButton.Location = new Point(100, 100);
            _mCheckButton.Click += new EventHandler(_mCheck_Click);

            // create assignment button
            _mAssignButton.Size = new Size(100, 30);
            _mAssignButton.Text = "Assign";
            _mAssignButton.Location = new Point(100, 135);
            _mAssignButton.Click += new EventHandler(_mAssignButton_Click);

            // create a combo box
            _mComboBox = new ComboBox();
            _mComboBox.Size = new System.Drawing.Size(300, 30);
            _mComboBox.Location = new Point(100, 200);

            this.Controls.AddRange(new Control[] {
                _mComboBox,
                _mCheckButton,
                _mAssignButton
            });

            // fill the bindinglist
            _mBindingList.Add(new OptionValue("One", 1M));
            _mBindingList.Add(new OptionValue("Two", 2M));
            _mBindingList.Add(new OptionValue("Three", 3M));

            _mCacheList.Add(new OptionValue("One", 1M));
            _mCacheList.Add(new OptionValue("Two", 2M));
            _mCacheList.Add(new OptionValue("Three", 3M));
        }

        void _mAssignButton_Click(object sender, EventArgs e)
        {
            // reset options
            _mBindingList.Clear();
            foreach (var o in _mCacheList)
                _mBindingList.Add(o);

            // EXPECTED: Update ComboBox.SelectedValue and ComboBox.Text
            // RESULT: Does not happen.
            _mEntity.WifeCount = 3M;

            this.Text = string.Format("SelectedValue: {0}; WifeCount: {1}", _mComboBox.SelectedValue, _mEntity.WifeCount);
        }

        private void PrepareComboBox(ComboBox combobox, BindingList<OptionValue> list)
        {
            combobox.DropDownStyle = ComboBoxStyle.DropDown;
            combobox.AutoCompleteSource = AutoCompleteSource.ListItems;
            combobox.AutoCompleteMode = AutoCompleteMode.Suggest;
            combobox.DataSource = new BindingSource() { DataSource = list };
            combobox.DisplayMember = "Display";
            combobox.ValueMember = "Value";
            combobox.Text = string.Empty;
            combobox.SelectedText = string.Empty;
        }

        protected override void OnLoad(EventArgs e)
        {
            // combo box datasource binding
            PrepareComboBox(_mComboBox, _mBindingList);

            // entity data binding
            _mComboBox.DataBindings.Add("SelectedValue", _mEntity, "WifeCount", false);

            base.OnLoad(e);
        }

        void _mCheck_Click(object sender, EventArgs e)
        {
            this.Text = string.Format("SelectedValue: {0}; WifeCount: {1}", _mComboBox.SelectedValue, _mEntity.WifeCount);
        }
    }

    public class Entity : INotifyPropertyChanged
    {
        decimal _mWifeCount;

        public decimal WifeCount { get { return _mWifeCount; } set { _mWifeCount = value; OnPropertyChanged("WifeCount"); } }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public class OptionValue
    {
        string _mDisplay;
        object _mValue;

        public string Display { get { return _mDisplay; } set { _mDisplay = value; } }

        public object Value { get { return _mValue; } set { _mValue = value; } }

        public OptionValue(string display, object value)
        {
            _mDisplay = display;
            _mValue = value;
        }
    }
}

更新：向ComboBox添加事件处理程序似乎有效：

void _mComboBox_SelectedValueChanged(object sender, EventArgs e)
{
      var binding = (sender as Control).DataBindings["SelectedValue"];
      if (binding != null)
          binding.WriteValue();

      this.Text = string.Format("SelectedValue: {0}; WifeCount: {1}", _mComboBox.SelectedValue, _mEntity.WifeCount);
}

Answer 1

我相信为了进行双向绑定，您需要为绑定列表中的元素实现INotifyPropertyChanged接口。原因是被用作数据源的BindingList不知道任何元素何时发生了变化，除非元素传递了该信息。但是，它仍然可以传递与添加和删除项目相关的事件（假设您将AllowRemove / AllowNew属性指定为true），因为该事件位于列表的范围内，而不是单个元素。

编辑：呸！跳了枪，没有彻底阅读问题/问题。这是问题，添加数据绑定显然默认为单向绑定（仅限初始绑定值）。您需要做的是在将数据绑定添加到组合框时指定DataSourceUpdateMode：

_mComboBox.DataBindings.Add("SelectedValue", _mEntity, "WifeCount", false, DataSourceUpdateMode.OnPropertyChanged);

刚刚测试了这一切，其他一切都保持不变并且有效。让我知道！

编辑：所以它不起作用（我没有清除列表），我想出了原因。所以这就是我注意到的。出于某种原因，只要基础数据源发生变化，实体的绑定上下文就会被清除。不完全确定原因，但我绝对发现这就是问题所在。我发现的方法是在实体的_mComboBox的绑定上下文中添加一个监视：_mComboBox.BindingContext[_mEntity]并跟踪绑定计数。一旦新项目被添加到_mBindingList，它似乎与ComboBox的内部数据绑定混乱，最终丢弃了Entity.WifeCount的绑定 - ＆gt;我们设置的ComboBox.SelectedValue绑定。试过各种各样的事情，但我不完全确定为什么PropertyManager在基础数据源发生变化时会删除绑定。

Answer 2

既然我明白你要做什么，我认为这可能是一个可行的解决方案：

双向绑定很好，但是清除组合框的数据源也会导致数据绑定。如果您要更改绑定列表，则应该在数据源发生更改时重新绑定：

protected override void OnLoad(EventArgs e)
{
    // combo box datasource binding
    PrepareComboBox(_mComboBox, _mBindingList);

    // entity data binding
    UpdateBindings();

    base.OnLoad(e);
}


public void UpdateBindings()
{
    _mComboBox.DataBindings.Clear();
    if (_mBindingList.Count != 0) _mComboBox.DataBindings.Add("SelectedValue", _mEntity, "WifeCount");
}

void _mAssignButton_Click(object sender, EventArgs e)
{
    _mBindingList.Clear();          
    foreach (var o in _mCacheList) _mBindingList.Add(o);
    // UPDATE BINDINGS HERE - Only do this if changing the binding source
    UpdateBindings();

    _mEntity.WifeCount = 3M;

    this.Text = string.Format("SelectedValue: {0}; WifeCount: {1}", _mComboBox.SelectedValue, _mEntity.WifeCount);
}

为什么在BindingList更改时会清除ComboBox.SelectedValue DataBinding上下文？

2 个答案: