尝试此类

参考

using Microsoft.VisualBasic;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
public class FlatCombo : ComboBox
{

    private Brush BorderBrush = new SolidBrush(SystemColors.Window);
    private Brush ArrowBrush = new SolidBrush(SystemColors.ControlText);
    private Brush DropButtonBrush = new SolidBrush(SystemColors.Control);

    private Color _ButtonColor = SystemColors.Control;
    public Color ButtonColor {
        get { return _ButtonColor; }
        set {
            _ButtonColor = value;
            DropButtonBrush = new SolidBrush(this.ButtonColor);
            this.Invalidate();
        }
    }

    protected override void WndProc(ref Message m)
    {
        base.WndProc(m);

        switch (m.Msg) {
            case 0xf:
                //Paint the background. Only the borders
                //will show up because the edit
                //box will be overlayed
                Graphics g = this.CreateGraphics;
                Pen p = new Pen(Color.White, 2);
                g.FillRectangle(BorderBrush, this.ClientRectangle);

                //Draw the background of the dropdown button
                Rectangle rect = new Rectangle(this.Width - 15, 3, 12, this.Height - 6);
                g.FillRectangle(DropButtonBrush, rect);

                //Create the path for the arrow
                Drawing2D.GraphicsPath pth = new Drawing2D.GraphicsPath();
                PointF TopLeft = new PointF(this.Width - 13, (this.Height - 5) / 2);
                PointF TopRight = new PointF(this.Width - 6, (this.Height - 5) / 2);
                PointF Bottom = new PointF(this.Width - 9, (this.Height + 2) / 2);
                pth.AddLine(TopLeft, TopRight);
                pth.AddLine(TopRight, Bottom);

                g.SmoothingMode = Drawing2D.SmoothingMode.HighQuality;

                //Determine the arrow's color.
                if (this.DroppedDown) {
                    ArrowBrush = new SolidBrush(SystemColors.HighlightText);
                } else {
                    ArrowBrush = new SolidBrush(SystemColors.ControlText);
                }

                //Draw the arrow
                g.FillPath(ArrowBrush, pth);

                break;
            default:
                break; // TODO: might not be correct. Was : Exit Select

                break;
        }
    }

    //Override mouse and focus events to draw
    //proper borders. Basically, set the color and Invalidate(),
    //In general, Invalidate causes a control to redraw itself.
    #region "Mouse and focus Overrides"
    protected override void OnMouseEnter(System.EventArgs e)
    {
        base.OnMouseEnter(e);
        BorderBrush = new SolidBrush(SystemColors.Highlight);
        this.Invalidate();
    }

    protected override void OnMouseLeave(System.EventArgs e)
    {
        base.OnMouseLeave(e);
        if (this.Focused)
            return;
        BorderBrush = new SolidBrush(SystemColors.Window);
        this.Invalidate();
    }

    protected override void OnLostFocus(System.EventArgs e)
    {
        base.OnLostFocus(e);
        BorderBrush = new SolidBrush(SystemColors.Window);
        this.Invalidate();
    }

    protected override void OnGotFocus(System.EventArgs e)
    {
        base.OnGotFocus(e);
        BorderBrush = new SolidBrush(SystemColors.Highlight);
        this.Invalidate();
    }

    protected override void OnMouseHover(System.EventArgs e)
    {
        base.OnMouseHover(e);
        BorderBrush = new SolidBrush(SystemColors.Highlight);
        this.Invalidate();
    }
    #endregion
}