具有文本支持的ComboBox DropDownList

Question

我创建了一个ComboBox，当DropDownStyle设置为DropDownList时，它支持在控件上绘制文本。

下面的代码段与代码结构的其余部分不匹配，也没有使用e.States中的正确检查。有谁知道我应该检查哪个e.States？

if (!this.DroppedDown && e.Index == -1)
        {
            e.Graphics.DrawString(_dropDownListText, e.Font, b, e.Bounds);
        }
        else if (e.Index > -1)
        {
            e.Graphics.DrawString(ItemName(e.Index), e.Font, b, e.Bounds);
        }

完整的代码，上面的代码段已经注释掉了以便于搜索

using System.ComponentModel;

public class ComboBoxEx : ComboBox
{
    private ToolTip _toolTip = new ToolTip();
    private string _dropDownListText = "Select...";

    public ComboBoxEx()
    {
        this.DrawMode = DrawMode.OwnerDrawVariable;
        this.DropDownStyle = ComboBoxStyle.DropDownList;
    }

    [Description("Gets or sets the text associated with DropDownList style."), Category("Appearance")]
    public string DropDownListText
    {
        get
        {
            return _dropDownListText;
        }
        set
        {
            if (string.IsNullOrEmpty(value))
            {
                value = string.Empty;
            }
            _dropDownListText = value;
        }
    }

    public string ItemName(int index)
    {
        if (index == -1)
        {
            return string.Empty;
        }
        // return the selected item value from the ComboBoxEx.Item class
        return ((ComboBoxEx.Item)(this.Items[index])).Name;
    }

    public string ItemTag(int index)
    {
        if (index == -1)
        {
            return string.Empty;
        }
        // return the selected item tag from the ComboBoxEx.Item class
        return ((ComboBoxEx.Item)(this.Items[index])).Tag;
    }

    protected override void OnSelectedIndexChanged(System.EventArgs e)
    {
        // force a redraw of DropDownListText if necessary
        base.OnSelectedIndexChanged(e);
        base.Invalidate();
    }

    protected override void OnDrawItem(System.Windows.Forms.DrawItemEventArgs e)
    {

        // select the brush to use as the fore color
        SolidBrush b = new SolidBrush((this.Enabled ? SystemColors.WindowText : SystemColors.GrayText));

        if ((e.State & DrawItemState.Focus) == DrawItemState.Focus)
        {
            // fill the focused items rectangle
            e.Graphics.FillRectangle(new SolidBrush(SystemColors.Info), e.Bounds);
        }
        else
        {
            // fill the rest of the rectangle
            e.Graphics.FillRectangle(new SolidBrush(SystemColors.Window), e.Bounds);
        }

// for ease of reading, I've commented the code out. This code should use proper if then e.State checks so that I can combine it with the rest of the Else If Then statements
        /* 
        if (!this.DroppedDown && e.Index == -1)
        {
            // redraw the dropdownlist text when the combobox is closed with no index selected
            e.Graphics.DrawString(_dropDownListText, e.Font, b, e.Bounds);
        }
        else if (e.Index > -1)
        {
            // else, draw the selected item value
            e.Graphics.DrawString(ItemName(e.Index), e.Font, b, e.Bounds);
        }
*/
        if (e.State == (DrawItemState.Disabled | DrawItemState.NoAccelerator | DrawItemState.NoFocusRect | DrawItemState.ComboBoxEdit))
        {
            // redraw the dropdownlist text when the control is disabled
            e.Graphics.DrawString(_dropDownListText, e.Font, b, 3, 3);
        }
        else if (e.State == (DrawItemState.NoAccelerator | DrawItemState.NoFocusRect | DrawItemState.ComboBoxEdit) && e.Index == -1)
        {
            // redraw the dropdownlist text when the control is dropped down with no index selected
            e.Graphics.DrawString(_dropDownListText, e.Font, b, 3, 3);
        }
        else if ((e.State & DrawItemState.Selected) == DrawItemState.Selected)
        {
            if (e.Index > -1 && this.DroppedDown)
            {
                // show a tool tip on the right side of the control with the selected item tag
                _toolTip.Show(ItemTag(e.Index), this, e.Bounds.Right, e.Bounds.Bottom + 5);
            }
            else
            {
                // hide the tool tip when the control is no longer dropped down
                _toolTip.Hide(this);
            }
        }

        e.DrawFocusRectangle();
        b.Dispose();

    }

    public class Item : object
    {
        protected string _name;
        protected string _tag;

        public Item(string name, string tag)
        {
            _name = name;
            _tag = tag;
        }

        public string Name
        {
            get
            {
                return _name;
            }
        }

        public string Tag
        {
            get
            {
                return _tag;
            }
        }

    }

}

使用示例：

    ComboBoxEx1.Items.Add(new ComboBoxEx.Item("Marks Number", "1-555-555-1000"));
    ComboBoxEx1.Items.Add(new ComboBoxEx.Item("Jacks Number", "1-555-555-1001"));
    ComboBoxEx1.Items.Add(new ComboBoxEx.Item("Daves Number", "1-555-555-1002"));
    ComboBoxEx1.Items.Add(new ComboBoxEx.Item("Joses Number", "1-555-555-1003"));

随意使用此代码供您自己使用。