我自己也遇到了这个问题，pepoluan的回复让我开始了。我仍然认为为了获得一个外观和行为类似于具有 DropDownStyle=DropDownList 的标准 ComboBox 的 ComboBox 缺少一些东西。
DropDownArrow
我们还需要绘制 DropDownArrow。我玩过 ComboBoxRenderer，但它在下拉箭头区域周围绘制了一个黑色边框，因此不起作用。
我的最终解决方案是简单地绘制一个类似的箭头并将其渲染到 OnPaint 方法中的按钮上。
热项行为
我们还需要确保我们的 ComboBox 具有类似于标准 ComboBox 的热项行为。我不知道有什么简单可靠的方法可以知道鼠标何时不再位于控件之上。因此，我建议使用 Timer 来检查每次滴答时鼠标是否仍在控件上方。
编辑
刚刚添加了一个 KeyUp 事件处理程序，以确保在使用键盘进行选择时控件将正确更新。还对文本的呈现位置进行了小幅修正，以确保它更类似于香草组合框的文本定位。
下面是我定制的 ComboBox 的完整代码。它允许您在每个项目上显示图像，并且始终以 DropDownList 样式呈现，但希望它应该很容易将代码适应您自己的解决方案。
using System;
using System.ComponentModel;
using System.Drawing;
using System.Drawing.Design;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Windows.Forms;
using System.Windows.Forms.VisualStyles;
namespace CustomControls
{
/// <summary>
/// This is a special ComboBox that each item may conatins an image.
/// </summary>
public class ImageComboBox : ComboBox
{
private static readonly Size arrowSize = new Size(18, 20);
private bool itemIsHot;
/* Since properties such as SelectedIndex and SelectedItems may change when the mouser is hovering over items in the drop down list
* we need a property that will store the item that has been selected by comitted selection so we know what to draw as the selected item.*/
private object comittedSelection;
private readonly ImgHolder dropDownArrow = ImgHolder.Create(ImageComboBox.DropDownArrow());
private Timer hotItemTimer;
public Font SelectedItemFont { get; set; }
public Padding ImageMargin { get; set; }
//
// Summary:
// Gets or sets the path of the property to use as the image for the items
// in the System.Windows.Forms.ListControl.
//
// Returns:
// A System.String representing a single property name of the System.Windows.Forms.ListControl.DataSource
// property value, or a hierarchy of period-delimited property names that resolves
// to a property name of the final data-bound object. The default is an empty string
// ("").
//
// Exceptions:
// T:System.ArgumentException:
// The specified property path cannot be resolved through the object specified by
// the System.Windows.Forms.ListControl.DataSource property.
[DefaultValue("")]
[Editor("System.Windows.Forms.Design.DataMemberFieldEditor, System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
public string ImageMember { get; set; }
public ImageComboBox()
{
base.SetStyle(ControlStyles.Opaque | ControlStyles.UserPaint, true);
//All the elements in the control are drawn manually.
base.DrawMode = DrawMode.OwnerDrawFixed;
//Specifies that the list is displayed by clicking the down arrow and that the text portion is not editable.
//This means that the user cannot enter a new value.
//Only values already in the list can be selected.
this.DropDownStyle = ComboBoxStyle.DropDownList;
//using DrawItem event we need to draw item
this.DrawItem += this.ComboBoxDrawItemEvent;
this.hotItemTimer = new Timer();
this.hotItemTimer.Interval = 250;
this.hotItemTimer.Tick += this.HotItemTimer_Tick;
this.MouseEnter += this.ImageComboBox_MouseEnter;
this.KeyUp += this.ImageComboBox_KeyUp;
this.SelectedItemFont = this.Font;
this.ImageMargin = new Padding(4, 4, 5, 4);
this.SelectionChangeCommitted += this.ImageComboBox_SelectionChangeCommitted;
this.SelectedIndexChanged += this.ImageComboBox_SelectedIndexChanged;
}
private static Image DropDownArrow()
{
var arrow = new Bitmap(8, 4, PixelFormat.Format32bppArgb);
using (Graphics g = Graphics.FromImage(arrow))
{
g.CompositingQuality = CompositingQuality.HighQuality;
g.FillPolygon(Brushes.Black, ImageComboBox.CreateArrowHeadPoints());
}
return arrow;
}
private static PointF[] CreateArrowHeadPoints()
{
return new PointF[4] { new PointF(0, 0), new PointF(7F, 0), new PointF(3.5F, 3.5F), new PointF(0, 0) };
}
private static void DrawComboBoxItem(Graphics g, string text, Image image, Rectangle itemArea, int itemHeight, int itemWidth, Padding imageMargin
, Brush brush, Font font)
{
if (image != null)
{
// recalculate margins so image is always approximately vertically centered
int extraImageMargin = itemHeight - image.Height;
int imageMarginTop = Math.Max(imageMargin.Top, extraImageMargin / 2);
int imageMarginBotttom = Math.Max(imageMargin.Bottom, extraImageMargin / 2);
g.DrawImage(image, itemArea.X + imageMargin.Left, itemArea.Y + imageMarginTop, itemHeight, itemHeight - (imageMarginBotttom
+ imageMarginTop));
}
const double TEXT_MARGIN_TOP_PROPORTION = 1.1;
const double TEXT_MARGIN_BOTTOM_PROPORTION = 2 - TEXT_MARGIN_TOP_PROPORTION;
int textMarginTop = (int)Math.Round((TEXT_MARGIN_TOP_PROPORTION * itemHeight - g.MeasureString(text, font).Height) / 2.0, 0);
int textMarginBottom = (int)Math.Round((TEXT_MARGIN_BOTTOM_PROPORTION * itemHeight - g.MeasureString(text, font).Height) / 2.0, 0);
//we need to draw the item as string because we made drawmode to ownervariable
g.DrawString(text, font, brush, new RectangleF(itemArea.X + itemHeight + imageMargin.Left + imageMargin.Right, itemArea.Y + textMarginTop
, itemWidth, itemHeight - textMarginBottom));
}
private string GetDistplayText(object item)
{
if (this.DisplayMember == string.Empty) { return item.ToString(); }
else
{
var display = item.GetType().GetProperty(this.DisplayMember).GetValue(item).ToString();
return display ?? item.ToString();
}
}
private Image GetImage(object item)
{
if (this.ImageMember == string.Empty) { return null; }
else { return item.GetType().GetProperty(this.ImageMember).GetValue(item) as Image; }
}
private void ImageComboBox_SelectionChangeCommitted(object sender, EventArgs e)
{
this.comittedSelection = this.Items[this.SelectedIndex];
}
private void HotItemTimer_Tick(object sender, EventArgs e)
{
if (!this.RectangleToScreen(this.ClientRectangle).Contains(Cursor.Position)) { this.TurnOffHotItem(); }
}
private void ImageComboBox_KeyUp(object sender, KeyEventArgs e)
{
this.Invalidate();
}
private void ImageComboBox_MouseEnter(object sender, EventArgs e)
{
this.TurnOnHotItem();
}
private void ImageComboBox_SelectedIndexChanged(object sender, EventArgs e)
{
if (!this.DroppedDown)
{
if (this.SelectedIndex > -1) { this.comittedSelection = this.Items[this.SelectedIndex]; }
else { this.comittedSelection = null; }
}
}
private void TurnOnHotItem()
{
this.itemIsHot = true;
this.hotItemTimer.Enabled = true;
}
private void TurnOffHotItem()
{
this.itemIsHot = false;
this.hotItemTimer.Enabled = false;
this.Invalidate(this.ClientRectangle);
}
/// <summary>
/// Draws overridden items.
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void ComboBoxDrawItemEvent(object sender, DrawItemEventArgs e)
{
//Draw backgroud of the item
e.DrawBackground();
if (e.Index != -1)
{
Brush brush;
if (e.State.HasFlag(DrawItemState.Focus) || e.State.HasFlag(DrawItemState.Selected)) { brush = Brushes.White; }
else { brush = Brushes.Black; }
object item = this.Items[e.Index];
ImageComboBox.DrawComboBoxItem(e.Graphics, this.GetDistplayText(item), this.GetImage(item), e.Bounds, this.ItemHeight, this.DropDownWidth
, new Padding(0, 1, 5, 1), brush, this.Font);
}
}
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
// define the area of the control where we will write the text
var topTextRectangle = new Rectangle(e.ClipRectangle.X - 1, e.ClipRectangle.Y - 1, e.ClipRectangle.Width + 2, e.ClipRectangle.Height + 2);
using (var controlImage = new Bitmap(e.ClipRectangle.Width, e.ClipRectangle.Height, PixelFormat.Format32bppArgb))
{
using (Graphics ctrlG = Graphics.FromImage(controlImage))
{
/* Render the control. We use ButtonRenderer and not ComboBoxRenderer because we want the control to appear with the DropDownList style. */
if (this.DroppedDown) { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Pressed); }
else if (this.itemIsHot) { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Hot); }
else { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Normal); }
// Draw item, if any has been selected
if (this.comittedSelection != null)
{
ImageComboBox.DrawComboBoxItem(ctrlG, this.GetDistplayText(this.comittedSelection), this.GetImage(this.comittedSelection)
, topTextRectangle, this.Height, this.Width - ImageComboBox.arrowSize.Width, this.ImageMargin, Brushes.Black, this.SelectedItemFont);
}
/* Now we need to draw the arrow. If we use ComboBoxRenderer for this job, it will display a distinct border around the dropDownArrow and we don't want that. As an alternative we define the area where the arrow should be drawn, and then procede to draw it. */
var dropDownButtonArea = new RectangleF(topTextRectangle.X + topTextRectangle.Width - (ImageComboBox.arrowSize.Width
+ this.dropDownArrow.Image.Width) / 2.0F, topTextRectangle.Y + topTextRectangle.Height - (topTextRectangle.Height
+ this.dropDownArrow.Image.Height) / 2.0F, this.dropDownArrow.Image.Width, this.dropDownArrow.Image.Height);
ctrlG.DrawImage(this.dropDownArrow.Image, dropDownButtonArea);
}
if (this.Enabled) { e.Graphics.DrawImage(controlImage, 0, 0); }
else { ControlPaint.DrawImageDisabled(e.Graphics, controlImage, 0, 0, Color.Transparent); }
}
}
}
internal struct ImgHolder
{
internal Image Image
{
get
{
return this._image ?? new Bitmap(1, 1); ;
}
}
private Image _image;
internal ImgHolder(Bitmap data)
{
_image = data;
}
internal ImgHolder(Image data)
{
_image = data;
}
internal static ImgHolder Create(Image data)
{
return new ImgHolder(data);
}
internal static ImgHolder Create(Bitmap data)
{
return new ImgHolder(data);
}
}
}
