这是一个从Control派生的自定义控件，它可以设置为半透明。
界面是一个彩色圆圈，可以包含几个数字。
该控件公开这些自定义属性：

Opacity：控件的不透明度级别BackGround[0, 255]
InnerPadding：内部矩形（定义圆边界）与控件边界之间的距离。
FontPadding：文本和内部矩形之间的距离。

通过覆盖CreateParams，然后设置ExStyle |= WS_EX_TRANSPARENT;获得透明度
Control.SetStyle()方法用于修改控件行为，添加以下ControlStyles：
▶**ControlStyles.Opaque：防止绘制控件的背景，因此它不受系统管理。结合CreateParams将控件的扩展样式设置为WS_EX_TRANSPARENT，控件将变得完全透明。
▶ControlStyles.SupportsTransparentBackColor该控件的BackGround颜色接受Alpha值。如果不设置ControlStyles.UserPaint，它将不会被用来模拟透明度。我们正在使用其他方法来模拟透明度。
要查看它的工作情况，请创建一个新的类文件，用此代码替换其中的所有代码并保留命名空间**，然后构建项目/解决方案。
新的自定义控件将出现在工具箱中。将其拖放到窗体上。根据需要修改其自定义属性。
控件的可视化表示形式：

注意事项和免责声明：

• 这是一个原型控件，缺少自定义设计器（不能在这里发布，代码太多，也连接到一个框架）。

正如这里所介绍的，它可以用来完全重叠表单或其他容器中的其他控件。在这个简化的实现中，不处理部分重叠。

• 字体被硬编码为Segoe UI，因为该字体有一个基线，简化了文本在圆形区域中间的位置。

其他字体具有不同的基线，这需要更复杂的处理。
有关基本数学，请参见：TextBox with dotted lines for typing。

using System;
using System.ComponentModel;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Text;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows.Forms;

[DesignerCategory("Code")]
public class RoundCenterLabel : Label, INotifyPropertyChanged, ISupportInitialize 
{
    private const int WS_EX_TRANSPARENT = 0x00000020;
    private bool IsInitializing = false;
    private Point MouseDownLocation = Point.Empty;
    private readonly int fontPadding = 4;
    private Font m_CustomFont = null;
    private Color m_BackGroundColor;
    private int m_InnerPadding = 0;
    private int m_FontPadding = 25;
    private int m_Opacity = 128;

    public event PropertyChangedEventHandler PropertyChanged;

    public RoundCenterLabel() => InitializeComponent();

    private void InitializeComponent()
    {
        SetStyle(ControlStyles.Opaque |
                 ControlStyles.SupportsTransparentBackColor |
                 ControlStyles.ResizeRedraw, true);
        SetStyle(ControlStyles.OptimizedDoubleBuffer, false);
        m_CustomFont = new Font("Segoe UI", 50, FontStyle.Regular, GraphicsUnit.Pixel);
        BackColor = Color.LimeGreen;
        ForeColor = Color.White;
    }

    protected override CreateParams CreateParams {
        get {
            var cp = base.CreateParams;
            cp.ExStyle |= WS_EX_TRANSPARENT;
            return cp;
        }
    }

    public new Font Font
    {
        get => m_CustomFont;
        set { 
            m_CustomFont = value;
            if (IsInitializing) return;
            FontAdapter(value, DeviceDpi);
            NotifyPropertyChanged();
        }
    }

    public override string Text {
        get => base.Text;
        set { base.Text = value;
              NotifyPropertyChanged();
        }
    }

    public int InnerPadding {
        get => m_InnerPadding;
        set {
            if (IsInitializing) return;
            m_InnerPadding = ValidateRange(value, 0, ClientRectangle.Height - 10);
            NotifyPropertyChanged(); }
    }

    public int FontPadding {
        get => m_FontPadding;
        set {
            if (IsInitializing) return;
            m_FontPadding = ValidateRange(value, 0, ClientRectangle.Height - 10);
            NotifyPropertyChanged();
        }
    }

    public int Opacity {
        get => m_Opacity;
        set { m_Opacity = ValidateRange(value, 0, 255);
              UpdateBackColor(m_BackGroundColor);
              NotifyPropertyChanged();
        }
    }

    public override Color BackColor {
        get => m_BackGroundColor;
        set { UpdateBackColor(value);
              NotifyPropertyChanged();
        }
    }

    protected override void OnLayout(LayoutEventArgs e)
    {
        base.OnLayout(e);
        base.AutoSize = false;
    }

    private void NotifyPropertyChanged([CallerMemberName] string PropertyName = null)
    {
        InvalidateParent();
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(PropertyName));
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);
        MouseDownLocation = e.Location;
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);
        if (e.Button == MouseButtons.Left) {
            var loc = new Point(Left + (e.X - MouseDownLocation.X), Top + (e.Y - MouseDownLocation.Y));
            InvalidateParent();
            BeginInvoke(new Action(() => Location = loc));
        }
    }

    private void InvalidateParent()
    {
        Parent?.Invalidate(Bounds, true);
        Invalidate();
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        using (var format = new StringFormat(StringFormatFlags.LineLimit | StringFormatFlags.NoWrap, CultureInfo.CurrentUICulture.LCID))
        {
            format.LineAlignment = StringAlignment.Center;
            format.Alignment = StringAlignment.Center;
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            e.Graphics.TextRenderingHint = TextRenderingHint.AntiAliasGridFit;

            using (var circleBrush = new SolidBrush(m_BackGroundColor))
            using (var foreBrush = new SolidBrush(ForeColor))
            {
                FontAdapter(m_CustomFont, e.Graphics.DpiY);
                RectangleF rect = InnerRectangle();
                e.Graphics.FillEllipse(circleBrush, rect);
                e.Graphics.DrawString(Text, m_CustomFont, foreBrush, rect, format);
            };
        };
    }

    public void BeginInit() => IsInitializing = true;

    public void EndInit()
    {
        IsInitializing = false;
        Font = new Font("Segoe UI", 50, FontStyle.Regular, GraphicsUnit.Pixel);
        FontPadding = m_FontPadding;
        InnerPadding = m_InnerPadding;
    }

    private RectangleF InnerRectangle()
    {
        (float Min, _) = GetMinMax(ClientRectangle.Height, ClientRectangle.Width);
        var size = new SizeF(Min - (m_InnerPadding / 2), Min - (m_InnerPadding / 2));
        var position = new PointF((ClientRectangle.Width - size.Width) / 2,
                                  (ClientRectangle.Height - size.Height) / 2);
        return new RectangleF(position, size);
    }

    private void FontAdapter(Font font, float dpi)
    {
        RectangleF rect = InnerRectangle();
        float fontSize = ValidateRange(
            (int)(rect.Height - m_FontPadding), 6, 
            (int)(rect.Height - m_FontPadding)) / (dpi / 72.0F) - fontPadding;

        m_CustomFont.Dispose();
        m_CustomFont = new Font(font.FontFamily, fontSize, font.Style, GraphicsUnit.Pixel);
    }

    private void UpdateBackColor(Color color)
    {
        m_BackGroundColor = Color.FromArgb(m_Opacity, Color.FromArgb(color.R, color.G, color.B));
        base.BackColor = m_BackGroundColor;
    }

    private int ValidateRange(int Value, int Min, int Max) 
        => Math.Max(Math.Min(Value, Max), Min); // (Value < Min) ? Min : ((Value > Max) ? Max : Value);

    private (float, float) GetMinMax(float Value1, float Value2) 
        => (Math.Min(Value1, Value2), Math.Max(Value1, Value2));
}