拖动picturebox上的图片，上层的透明控件闪烁

透明控件代码

public partial class RoundButton : Control
    {
        public RoundButton()
        {
            this.SetStyle(ControlStyles.UserPaint, true);
            this.SetStyle(ControlStyles.ResizeRedraw, true);
            this.SetStyle(ControlStyles.AllPaintingInWmPaint, true);
            this.SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
            this.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
            this.BackColor = Color.Transparent;
            InitializeComponent();
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            g.DrawRectangle(Pens.DarkCyan, new Rectangle(0, 0, this.Width - 1, this.Height - 1));
            using (SolidBrush sb = new SolidBrush(this.ForeColor))
            {
                SizeF se = g.MeasureString(this.Text, this.Font);
                g.DrawString(this.Text, this.Font, sb, (this.Width - se.Width) / 2, (this.Height - se.Height)/2);
            } 
            base.OnPaint(e);
        }

        protected override void OnPaintBackground(PaintEventArgs e) //不画背景
        {
            base.OnPaintBackground(e);  
        }

        protected override CreateParams CreateParams
        {
            get
            {
                CreateParams para = base.CreateParams;
                para.ExStyle |= 0x20; //WS_EX_TRANSPARENT 透明支持
                return para;
            }
        } 
    } 

拖动图片时，透明控件会消失，静止才出现。

https://www.cnblogs.com/MartianLan/p/5013075.html