c# - 在 BindingList 的 ItemChanging 事件中获取 Deleted Item

Question

我在我的应用程序中使用绑定(bind)列表以及 ItemChanged 事件。

有什么方法可以让我知道 ItemChanged 事件中属性的先前值。目前，我正在添加一个名为“OldValue”的单独属性来实现此目的。

有没有办法知道项目更改事件中已删除的项目。我无法找到任何方法来知道哪个项目已从列表中删除。

Answer 1

如果我没理解错的话，你想获取有关已从绑定(bind)列表中删除的项目的信息。

我认为最简单的方法是创建您自己的绑定(bind)列表，该绑定(bind)列表派生自绑定(bind)列表。

在内部，您将覆盖 RemoveItem 方法，因此在从绑定(bind)列表中删除项目之前，您将能够触发包含将要删除的项目的事件。

public class myBindingList<myInt> : BindingList<myInt>
{
    protected override void RemoveItem(int itemIndex)
    {
        //itemIndex = index of item which is going to be removed
        //get item from binding list at itemIndex position
        myInt deletedItem = this.Items[itemIndex];

        if (BeforeRemove != null)
        {
            //raise event containing item which is going to be removed
            BeforeRemove(deletedItem);
        }

        //remove item from list
        base.RemoveItem(itemIndex);
    }

    public delegate void myIntDelegate(myInt deletedItem);
    public event myIntDelegate BeforeRemove;
}

为了示例，我创建了实现 INotifyPropertyChanged 的​​类型 myInt - 接口(interface)只是为了在从绑定(bind)列表中添加/删除元素后刷新 dataGridView。

public class myInt : INotifyPropertyChanged
{
    public myInt(int myIntVal)
    {
        myIntProp = myIntVal;
    }
    private int iMyInt;
    public int myIntProp {
        get
        {
            return iMyInt;
        }
        set
        {
            iMyInt = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("myIntProp"));
            }
        } 
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

我正在使用 int 初始化绑定(bind)列表(准确地说是 myInts)，然后我将绑定(bind)列表绑定(bind)到 dataGridView(用于演示目的)并订阅我的 BeforeRemove 事件。

bindingList = new myBindingList<myInt>();
bindingList.Add(new myInt(8));
bindingList.Add(new myInt(9));
bindingList.Add(new myInt(11));
bindingList.Add(new myInt(12));

dataGridView1.DataSource = bindingList;
bindingList.BeforeRemove += bindingList_BeforeRemove;

如果引发了 BeforeRemove 事件，我有已删除的项目

void bindingList_BeforeRemove(Form1.myInt deletedItem)
{
    MessageBox.Show("You've just deleted item with value " + deletedItem.myIntProp.ToString());
}

下面是整个示例代码(在表单上放置 3 个按钮和 dataGridView)- 按钮 1 初始化绑定(bind)列表，按钮 2 将项目添加到列表，按钮 3 从投标列表中删除项目

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

namespace bindinglist
{
    public partial class Form1 : Form
    {
        myBindingList<myInt> bindingList;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            bindingList = new myBindingList<myInt>();
            bindingList.Add(new myInt(8));
            bindingList.Add(new myInt(9));
            bindingList.Add(new myInt(11));
            bindingList.Add(new myInt(12));

            dataGridView1.DataSource = bindingList;
            bindingList.BeforeRemove += bindingList_BeforeRemove;
        }

        void bindingList_BeforeRemove(Form1.myInt deletedItem)
        {
            MessageBox.Show("You've just deleted item with value " + deletedItem.myIntProp.ToString());
        }
        
        private void button2_Click(object sender, EventArgs e)
        {
            bindingList.Add(new myInt(13));
        }

        private void button3_Click(object sender, EventArgs e)
        {
            bindingList.RemoveAt(dataGridView1.SelectedRows[0].Index);
        }

        public class myInt : INotifyPropertyChanged
        {
            public myInt(int myIntVal)
            {
                myIntProp = myIntVal;
            }
            private int iMyInt;
            public int myIntProp {
                get
                {
                    return iMyInt;
                }
                set
                {
                    iMyInt = value;
                    if (PropertyChanged != null)
                    {
                        PropertyChanged(this, new PropertyChangedEventArgs("myIntProp"));
                    }
                } 
            }

            public event PropertyChangedEventHandler PropertyChanged;
        }

        public class myBindingList<myInt> : BindingList<myInt>
        {
            protected override void RemoveItem(int itemIndex)
            {
                myInt deletedItem = this.Items[itemIndex];

                if (BeforeRemove != null)
                {
                    BeforeRemove(deletedItem);
                }

                base.RemoveItem(itemIndex);
            }

            public delegate void myIntDelegate(myInt deletedItem);
            public event myIntDelegate BeforeRemove;
        }
    }
}

---

回复评论

“问题的另一部分是 => 有没有办法知道列表中更改的项目的旧值？在 ListChangedEvent 中不共享任何内容”

要查看项目的旧值，您可以覆盖 SetItem 方法

protected override void SetItem(int index, myInt item)
{
    //here we still have old value at index
    myInt oldMyInt = this.Items[index];
    //new value
    myInt newMyInt = item;

    if (myIntOldNew != null)
    {
        //raise event
        myIntOldNew(oldMyInt, newMyInt);
    }

    //update item at index position
    base.SetItem(index, item);
}

当指定索引处的对象发生更改时触发，如下所示

bindingList[dataGridView1.SelectedRows[0].Index] = new myInt(new Random().Next());

棘手的部分是，如果您尝试直接修改项目的属性

bindingList[dataGridView1.SelectedRows[0].Index].myIntProp = new Random().Next();

SetItem 不会触发，必须替换整个对象。

所以我们需要另一个委托(delegate)和事件来处理这个

public delegate void myIntDelegateChanged(myInt oldItem, myInt newItem);
public event myIntDelegateChanged myIntOldNew;

然后我们可以订阅这个

bindingList.myIntOldNew += bindingList_myIntOldNew;

并处理它

void bindingList_myIntOldNew(Form1.myInt oldItem, Form1.myInt newItem)
{
    MessageBox.Show("You've just CHANGED item with value " + oldItem.myIntProp.ToString() + " to " + newItem.myIntProp.ToString());
}

更新代码(需要 4 个按钮，第 4 个修改所选项目)

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

namespace bindinglist
{
    public partial class Form1 : Form
    {
        myBindingList<myInt> bindingList;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            bindingList = new myBindingList<myInt>();
            bindingList.Add(new myInt(8));
            bindingList.Add(new myInt(9));
            bindingList.Add(new myInt(11));
            bindingList.Add(new myInt(12));

            dataGridView1.DataSource = bindingList;
            bindingList.BeforeRemove += bindingList_BeforeRemove;
            bindingList.myIntOldNew += bindingList_myIntOldNew;
        }

        void bindingList_myIntOldNew(Form1.myInt oldItem, Form1.myInt newItem)
        {
            MessageBox.Show("You've just CHANGED item with value " + oldItem.myIntProp.ToString() + " to " + newItem.myIntProp.ToString());
        }

        void bindingList_BeforeRemove(Form1.myInt deletedItem)
        {
            MessageBox.Show("You've just deleted item with value " + deletedItem.myIntProp.ToString());
        }

        private void button2_Click(object sender, EventArgs e)
        {
            bindingList.Add(new myInt(13));
        }

        private void button3_Click(object sender, EventArgs e)
        {
            bindingList.RemoveAt(dataGridView1.SelectedRows[0].Index);
        }

        public class myInt : INotifyPropertyChanged
        {
            public myInt(int myIntVal)
            {
                myIntProp = myIntVal;
            }
            private int iMyInt;
            public int myIntProp {
                get
                {
                    return iMyInt;
                }
                set
                {
                    iMyInt = value;
                    if (PropertyChanged != null)
                    {
                        PropertyChanged(this, new PropertyChangedEventArgs("myIntProp"));
                    }
                } 
            }
            
            public event PropertyChangedEventHandler PropertyChanged;
        }

        public class myBindingList<myInt> : BindingList<myInt>
        {
            protected override void SetItem(int index, myInt item)
            {
                myInt oldMyInt = this.Items[index];
                myInt newMyInt = item;

                if (myIntOldNew != null)
                {
                    myIntOldNew(oldMyInt, newMyInt);
                }

                base.SetItem(index, item);
            }
            
            protected override void RemoveItem(int itemIndex)
            {
                myInt deletedItem = this.Items[itemIndex];

                if (BeforeRemove != null)
                {
                    BeforeRemove(deletedItem);
                }

                base.RemoveItem(itemIndex);
            }

            public delegate void myIntDelegateChanged(myInt oldItem, myInt newItem);
            public event myIntDelegateChanged myIntOldNew;

            public delegate void myIntDelegate(myInt deletedItem);
            public event myIntDelegate BeforeRemove;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            bindingList[dataGridView1.SelectedRows[0].Index] = new myInt(new Random().Next());
        }
    }
}