1、文件结构

2、控件视图

3、程序代码

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

namespace UserCtrlLib
{
    public partial class LatchPower : UserControl
    {
        public LatchPower()
        {
            InitializeComponent();
            RoundRadius = 10;
        }

        #region Properties
        private int roundRadius;
        [Description("圆角半径(0为不要圆角)")]
        [Category("自定义外观")]
        public int RoundRadius
        {
            get { return roundRadius; }
            set 
            { 
                if(value < 0)
                {
                    roundRadius = 0;
                }
                else
                {
                    roundRadius = value;
                }
                base.Refresh(); 
            }
        }

        #endregion

        #region Override Methods
        /// <summary>
        /// 在控件尺寸变化时刷新
        /// </summary>
        /// <param name="e"></param>
        protected override void OnResize(EventArgs e)
        {
            base.OnResize(e);
            base.Refresh();
        }
        /// <summary>
        /// 将控件边框修改成圆角
        /// </summary>
        /// <param name="e"></param>
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            Round(this.Region, RoundRadius);
        }
        #endregion

        #region UI Methods
        /// <summary>
        /// 绘制控件的圆角
        /// </summary>
        /// <param name="region"></param>
        /// <param name="radius"></param>
        public void Round(System.Drawing.Region region, int radius)
        {
            System.Drawing.Drawing2D.GraphicsPath oPath = new System.Drawing.Drawing2D.GraphicsPath();
            int x = 0;
            int y = 0;
            int thisWidth = this.Width;
            int thisHeight = this.Height;
            int angle = radius;
            // 半径非0为圆角矩形
            if (angle > 0)
            {
                System.Drawing.Graphics g = CreateGraphics();
                oPath.AddArc(x, y, angle, angle, 180, 90);                                 // 左上角
                oPath.AddArc(thisWidth - angle, y, angle, angle, 270, 90);                 // 右上角
                oPath.AddArc(thisWidth - angle, thisHeight - angle, angle, angle, 0, 90);  // 右下角
                oPath.AddArc(x, thisHeight - angle, angle, angle, 90, 90);                 // 左下角
                oPath.CloseAllFigures();
                Region = new System.Drawing.Region(oPath);
            }
            //半径为0时为矩形
            else
            {
                angle = 0;
                oPath.AddLine(x + angle, y, thisWidth - angle, y);                         // 顶端
                oPath.AddLine(thisWidth, y + angle, thisWidth, thisHeight - angle);        // 右边
                oPath.AddLine(thisWidth - angle, thisHeight, x + angle, thisHeight);       // 底边
                oPath.AddLine(x, y + angle, x, thisHeight - angle);                        // 左边
                oPath.CloseAllFigures();
                Region = new System.Drawing.Region(oPath);
            }
        }
        #endregion

    }
}

4、调用并运行