当我将鼠标悬停在 ComboBox 项目上时引发事件

Question

当我将鼠标悬停在我的 ComboBox 项上时，我找不到要触发的事件。
我正在使用 windows 表单构建应用程序。
我为 WPF 找到了类似的东西：
.

如何使用 Windows 表单中的类似方法，或者有其他方法吗？

Class ComboBoxListEx:

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Windows.Forms;

[DesignerCategory("Code")]
public class ComboBoxListEx : ComboBox
{
    private int listItem = -1;
    private const int CB_GETCURSEL = 0x0147;

    public event EventHandler<ListItemSelectionChangedEventArgs> ListItemSelectionChanged;

    protected virtual void OnListItemSelectionChanged(ListItemSelectionChangedEventArgs e)
        => this.ListItemSelectionChanged?.Invoke(this, e);

    public ComboBoxListEx() { }

    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        switch (m.Msg)
        {
            case CB_GETCURSEL:
                int selItem = m.Result.ToInt32();
                if (listItem != selItem)
                {
                    listItem = selItem;
                    OnListItemSelectionChanged(new ListItemSelectionChangedEventArgs(
                        listItem, listItem < 0 ? string.Empty : this.GetItemText(this.Items[listItem]))
                    );
                }
                break;
            default:
                // Add Case switches to handle other events
                break;
        }
    }

    public class ListItemSelectionChangedEventArgs : EventArgs
    {
        public ListItemSelectionChangedEventArgs(int idx, string text)
        {
            this.ItemIndex = idx;
            this.ItemText = text;
        }
        public int ItemIndex { get; private set; }
        public string ItemText { get; private set; }
    }
}         


private void comboBoxListEx1_ListItemSelectionChanged(object sender, ComboBoxListEx.ListItemSelectionChangedEventArgs e)
{
    label15.Text = e.ItemText;
}

Answer 1

您可以创建一个派生自 ComboBox 的自定义控件，覆盖其 WndProc method to intercept the CB_GETCURSEL 消息。

先打电话给base.WndProc(ref m)。处理消息时，Message 对象的 m.Result 属性被设置为一个值（如 IntPtr），表示当前在 ListBox 中跟踪的项目（当鼠标指针悬停在它上面）。

► 注意： 在 .Net Framework 4.8 之前，CB_GETCURSEL 消息结果不会 自动冒泡 ：LB_GETCUSEL 必须发送到子 ListBox 以获取当前 突出显示 的项目的索引。
使用 GetComboBoxInfo: it could be also accessed using reflection (the private ChildListAutomationObject property returns the ListBox AutomationElement, which provides the handle), or sending a CB_GETCOMBOBOXINFO 消息检索 ListBox 句柄（但它与调用 GetComboBoxInfo() 相同）。

此自定义 ComboBox 使用自定义 EventArgs 对象引发事件 ListItemSelectionChanged，ListItemSelectionChangedEventArgs，它公开了两个 public 属性：ItemIndex 和 ItemText，设置为索引和悬停项目的文本。

using System.ComponentModel;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

[DesignerCategory("Code")]
public class ComboBoxListEx : ComboBox
{
    private const int CB_GETCURSEL = 0x0147;
    private int listItem = -1;
    IntPtr listBoxHandle = IntPtr.Zero;

    public event EventHandler<ListItemSelectionChangedEventArgs> ListItemSelectionChanged;

    protected virtual void OnListItemSelectionChanged(ListItemSelectionChangedEventArgs e)
        => this.ListItemSelectionChanged?.Invoke(this, e);

    public ComboBoxListEx() { }

    // .Net Framework prior to 4.8 - get the handle of the ListBox
    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        listBoxHandle = GetComboBoxListInternal(this.Handle);
    }

    protected override void WndProc(ref Message m)
    {
        int selItem = -1;
        base.WndProc(ref m);

        switch (m.Msg) {
            case CB_GETCURSEL:
                selItem = m.Result.ToInt32();
                break;
            // .Net Framework prior to 4.8
            // case CB_GETCURSEL can be left there or removed: it's always -1
            case 0x0134: 
                selItem = SendMessage(listBoxHandle, LB_GETCUSEL, 0, 0);
                break;
            default:
                // Add Case switches to handle other events
                break;
        }
        if (listItem != selItem) {
            listItem = selItem;
            OnListItemSelectionChanged(new ListItemSelectionChangedEventArgs(
                listItem, listItem < 0 ? string.Empty : GetItemText(Items[listItem]))
            );
        }
    }

    public class ListItemSelectionChangedEventArgs : EventArgs
    {
        public ListItemSelectionChangedEventArgs(int idx, string text) {
            ItemIndex = idx;
            ItemText = text;
        }
        public int ItemIndex { get; private set; }
        public string ItemText { get; private set; }
    }

    // -------------------------------------------------------------
    // .Net Framework prior to 4.8
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    internal static extern bool GetComboBoxInfo(IntPtr hWnd, ref COMBOBOXINFO pcbi);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern int SendMessage(IntPtr hWnd, uint uMsg, int wParam, int lParam);
    
    private const int LB_GETCUSEL = 0x0188;

    [StructLayout(LayoutKind.Sequential)]
    internal struct COMBOBOXINFO
    {
        public int cbSize;
        public Rectangle rcItem;
        public Rectangle rcButton;
        public int buttonState;
        public IntPtr hwndCombo;
        public IntPtr hwndEdit;
        public IntPtr hwndList;
        public void Init() => this.cbSize = Marshal.SizeOf<COMBOBOXINFO>();
    }

    internal static IntPtr GetComboBoxListInternal(IntPtr cboHandle)
    {
        var cbInfo = new COMBOBOXINFO();
        cbInfo.Init();
        GetComboBoxInfo(cboHandle, ref cbInfo);
        return cbInfo.hwndList;
    }
}

像这样工作：

当我将鼠标悬停在 ComboBox 项目上时引发事件

Raise an event when I hover the mouse over a ComboBox item

c#

winforms

winapi

combobox

custom-controls