Xin chào các bạn, bài viết này mình chia sẻ các bạn đoạn code custom UI của Combobox thuần trên Winform C#.

[C#] How to custom combobox Winform

Khi custom combox lại chúng ta có thể chọn border color và caret trên combox.

Giao diện demo của combobox custom:

Đầu tiên, các bạn tạo 1 class với tên FlatCombobox.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace NoteWrapper
{
    internal 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);
    }
}

và sau đó, các bạn nhấn F5 để chạy ứng dụng.

Tiếp đến, các bạn mở Toolbox, sẽ thấy xuất hiện control FlatCombobox, và các bạn kéo ra sử dụng.

Thanks for watching!