C# 自定义控件导致 CS0436 警告和设计器不显示

Question

我正在开发一个项目，用于制作帖子并在其上打印任务。只是为了学习语言。我想将所有内容放入类中，并从这些类中构建项目。

因此，首先我在单独的文件中创建了一个名为 postit 的类。该文件是通过 VS2019 以某种方式导入到项目中的，因此我能够在工具箱中看到该控件。我可以很好地拖放便利贴，但一段时间后，设计器突然崩溃了。

如果我在调试器模式下运行代码并且其行为符合预期（选择、移动等），则代码运行良好。但是 VS2019 的设计器在以下错误消息中显示了一堆抱怨“错误 HRESULT E_FAIL 已从对 COM 组件的调用中返回。”，并在设计器所在的位置显示警告 CS0436代码调用类。据我了解，这是一种类型不匹配警告。这对我来说毫无意义，因为我只创建了一个地方，其中创建了名为 postit 的类型。这是如何运作的？

这是我的类代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.ComponentModel;

namespace Digipi.CustomControls
{
    public class Postit : Panel
    {
        //Fields
        public Color BGColor = ColorTranslator.FromHtml("#FFFFAA");
        private Color HoverColor = ColorTranslator.FromHtml("#FFAAAA");
        private Color LineColor = ColorTranslator.FromHtml("#AAAAFF");
        private int startLineX = 10;
        private int startLineY = 20;
        public int lineInterval = 2;
        public Color borderColor =Color.Black;
        public int nrLines=5;
        public int SelectionThickness =10;
        private Point MouseDownLocation;
        private bool moving = false;
        private bool selected = false;

        //Properties
        [Category("PostIt adjustments")]
        public Color BackgroundColor
        {
            get
            {
                return BGColor;
            }
            set
            {
                BGColor = value;
                this.Invalidate();
            }
        }
        [Category("PostIt adjustments")]
        public int SelectionBorder
        {
            get
            {
                return SelectionThickness;
            }
            set
            {
                SelectionThickness = value;
                this.Invalidate();
            }
        }
        //Constructor
        public Postit()
        {
            MinimumSize = new Size(50, 10);
            BackColor = BGColor;
        }

        //Methods
        private GraphicsPath Outline(bool border, float thickness=0)
        {
            GraphicsPath GP = new GraphicsPath();
            GP.StartFigure();
            GP.AddLine(thickness,thickness,this.Width-thickness,thickness);
            GP.AddLine(this.Width - thickness, thickness, this.Width - thickness, this.Height - thickness);
            GP.AddLine(this.Width - thickness, this.Height - thickness,thickness,this.Height-thickness);
            GP.CloseFigure();
            return GP;
        }

        protected override void OnClick(EventArgs e)
        {
            //base.OnClick(e);
            this.selected = true;
            /*foreach pi in this.Parent.Controls{

            }*/
            this.Invalidate();
        }
        protected override void OnMouseDown(MouseEventArgs e)
        {
            //base.OnMouseDown(e);
            if (this.moving)
            {
                return;
            }
            this.moving = true;
            this.MouseDownLocation = e.Location;
            

        }

        protected override void OnMouseMove(MouseEventArgs e)
        {
            base.OnMouseMove(e);
            if (!this.moving)
            {
                return;
            }
            //Point p = new Point(Cursor.Position.X - this.OffsetPoint.X, Cursor.Position.Y - this.OffsetPoint.Y);
            
            this.Left = e.X+this.Left-MouseDownLocation.X;
            this.Top = e.Y + this.Top - MouseDownLocation.Y;
        }

        protected override void OnMouseUp(MouseEventArgs e)
        {
            base.OnMouseUp(e);
            this.moving = false;
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            e.Graphics.Clear(Color.Transparent);
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            Brush b = new SolidBrush(BGColor);
            e.Graphics.FillPath(b, Outline(false));
            Pen pLine = new Pen(LineColor, 1);
            Pen pBorder = new Pen(borderColor, SelectionThickness);
            int lines = (int)(this.Height - startLineY )/ lineInterval;
            for (int i = 0; i < lines; i++)
            {
                e.Graphics.DrawLine(pLine, startLineX, startLineY+i*lineInterval, (float)(this.Width * 0.9), startLineY + i * lineInterval);
            }
            if (this.selected){
                e.Graphics.DrawPath(pBorder, Outline(true, SelectionThickness / 2));
            }

        }
    }

    
}

插入对象是通过设计器拖放完成的，因此这是标准的。

实在不明白设计师为啥这么崩溃。。