How to change the ComboBox dropdown button color

Question

How could I change the slide button color? not border color and not slide item colors.

I already change the slide item colors

Is there any way to change the color?

Answer 1

Flat ComboBox - Change border color and Dropdown button color

You need to handle WM_PAINT yourself and draw the border and the dropdown rectangle. This is the way that internal ComboBox.FlatComboAdapter class of .Net Framework works.

In this post, I've created a FlatComboBox, which draws the border and the dropdown in a flat style, having the following additional properties:

• BorderColor: used for border and for the dropdown arrow
• ButtonColor: used for dropdown area color.

Example - .NET Framework 4.8 and .NET 7.0

You can download or clone a working example for .NET Framework 4.8 and .NET 7.0 in the following links:

• Clone r-aghaei/FlatComboExample
• Download master.zip

Code

Here is the code of FlatComboBox:

using System;
using System.ComponentModel;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;
public class FlatComboBox : ComboBox
{
    private Color borderColor = Color.Gray;
    [DefaultValue(typeof(Color), "Gray")]
    public Color BorderColor
    {
        get { return borderColor; }
        set
        {
            if (borderColor != value)
            {
                borderColor = value;
                Invalidate();
            }
        }
    }
    private Color buttonColor = Color.LightGray;
    [DefaultValue(typeof(Color), "LightGray")]
    public Color ButtonColor
    {
        get { return buttonColor; }
        set
        {
            if (buttonColor != value)
            {
                buttonColor = value;
                Invalidate();
            }
        }
    }
    protected override void WndProc(ref Message m)
    {
        if (m.Msg == WM_PAINT && DropDownStyle != ComboBoxStyle.Simple)
        {
            var clientRect = ClientRectangle;
            var dropDownButtonWidth = SystemInformation.HorizontalScrollBarArrowWidth;
            var outerBorder = new Rectangle(clientRect.Location,
                new Size(clientRect.Width - 1, clientRect.Height - 1));
            var innerBorder = new Rectangle(outerBorder.X + 1, outerBorder.Y + 1,
                outerBorder.Width - dropDownButtonWidth - 2, outerBorder.Height - 2);
            var innerInnerBorder = new Rectangle(innerBorder.X + 1, innerBorder.Y + 1,
                innerBorder.Width - 2, innerBorder.Height - 2);
            var dropDownRect = new Rectangle(innerBorder.Right + 1, innerBorder.Y,
                dropDownButtonWidth, innerBorder.Height + 1);
            if (RightToLeft == RightToLeft.Yes)
            {
                innerBorder.X = clientRect.Width - innerBorder.Right;
                innerInnerBorder.X = clientRect.Width - innerInnerBorder.Right;
                dropDownRect.X = clientRect.Width - dropDownRect.Right;
                dropDownRect.Width += 1;
            }
            var innerBorderColor = Enabled ? BackColor : SystemColors.Control;
            var outerBorderColor = Enabled ? BorderColor : SystemColors.ControlDark;
            var buttonColor = Enabled ? ButtonColor : SystemColors.Control;
            var middle = new Point(dropDownRect.Left + dropDownRect.Width / 2,
                dropDownRect.Top + dropDownRect.Height / 2);
            var arrow = new Point[]
            {
                new Point(middle.X - 3, middle.Y - 2),
                new Point(middle.X + 4, middle.Y - 2),
                new Point(middle.X, middle.Y + 2)
            };
            var ps = new PAINTSTRUCT();
            bool shoulEndPaint = false;
            IntPtr dc;
            if (m.WParam == IntPtr.Zero)
            {
                dc = BeginPaint(Handle, ref ps);
                m.WParam = dc;
                shoulEndPaint = true;
            }
            else
            {
                dc = m.WParam;
            }
            var rgn = CreateRectRgn(innerInnerBorder.Left, innerInnerBorder.Top, 
                innerInnerBorder.Right, innerInnerBorder.Bottom);
            SelectClipRgn(dc, rgn);
            DefWndProc(ref m);
            DeleteObject(rgn);
            rgn = CreateRectRgn(clientRect.Left, clientRect.Top, 
                clientRect.Right, clientRect.Bottom);
            SelectClipRgn(dc, rgn);
            using (var g = Graphics.FromHdc(dc))
            {
                using (var b = new SolidBrush(buttonColor))
                {
                    g.FillRectangle(b, dropDownRect);
                }
                using (var b = new SolidBrush(outerBorderColor))
                {
                    g.FillPolygon(b, arrow);
                }
                using (var p = new Pen(innerBorderColor))
                {
                    g.DrawRectangle(p, innerBorder);
                    g.DrawRectangle(p, innerInnerBorder);
                }
                                    using (var p = new Pen(outerBorderColor))
                {
                    g.DrawRectangle(p, outerBorder);
                }
            }
            if (shoulEndPaint)
                EndPaint(Handle, ref ps);
            DeleteObject(rgn);
        }
        else
            base.WndProc(ref m);
    }

    private const int WM_PAINT = 0xF;
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int L, T, R, B;
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct PAINTSTRUCT
    {
        public IntPtr hdc;
        public bool fErase;
        public int rcPaint_left;
        public int rcPaint_top;
        public int rcPaint_right;
        public int rcPaint_bottom;
        public bool fRestore;
        public bool fIncUpdate;
        public int reserved1;
        public int reserved2;
        public int reserved3;
        public int reserved4;
        public int reserved5;
        public int reserved6;
        public int reserved7;
        public int reserved8;
    }
    [DllImport("user32.dll")]
    private static extern IntPtr BeginPaint(IntPtr hWnd,
        [In, Out] ref PAINTSTRUCT lpPaint);

    [DllImport("user32.dll")]
    private static extern bool EndPaint(IntPtr hWnd, ref PAINTSTRUCT lpPaint);

    [DllImport("gdi32.dll")]
    public static extern int SelectClipRgn(IntPtr hDC, IntPtr hRgn);

    [DllImport("user32.dll")]
    public static extern int GetUpdateRgn(IntPtr hwnd, IntPtr hrgn, bool fErase);
    public enum RegionFlags
    {
        ERROR = 0,
        NULLREGION = 1,
        SIMPLEREGION = 2,
        COMPLEXREGION = 3,
    }
    [DllImport("gdi32.dll")]
    internal static extern bool DeleteObject(IntPtr hObject);

    [DllImport("gdi32.dll")]
    private static extern IntPtr CreateRectRgn(int x1, int y1, int x2, int y2);
}

Related Posts

You may also want to take a look at the following flat style controls:

• Flat TextBox - Change border color of TextBox
• Flat DateTimePicker - Change border color and dropdown button of DateTimePicker
• Flat NumericUpDown - Change border color and spin buttons of NumericUpDown