一、起因

关于winform窗体无边框的问题很简单，只需要设置winform的窗体属性即可：

FormBorderStyle = FormBorderStyle.None;

但是这中无边框窗口实现的效果和背景完全没有层次的感觉，所以能加上阴影，突出窗口显示的感觉。

二、网上搜索的解决方案

方法 1

首先，发现了使用 user32.dll 中方法实现的方案：
C# WinForm无边框窗体设置阴影效果

缺点：这种方法只能实现右边和下边的阴影效果，效果不是很好。

效果如图：

方法 2

然后发现了使用双层窗体，底层窗体贴阴影的PNG来实现的方式：
教你实现Winform窗体的四边阴影效果

缺点：PNG格式特点是，支持alpha通道半透明，但是放大会失真

效果如图：

三、最终解决方案

我使用的方法是绘制阴影到bitmap上，然后使用双层窗体的原理把bitmap绘制到背景层上去。

其中比较重要的是：

1、绘制圆角矩形

public static void DrawRoundRectangle(Graphics g, Pen pen, Rectangle rect, int cornerRadius)
{using (GraphicsPath path = CreateRoundedRectanglePath(rect, cornerRadius)){g.DrawPath(pen, path);}
}
public static void FillRoundRectangle(Graphics g, Brush brush, Rectangle rect, int cornerRadius)
{using (GraphicsPath path = CreateRoundedRectanglePath(rect, cornerRadius)){g.FillPath(brush, path);}
}
internal static GraphicsPath CreateRoundedRectanglePath(Rectangle rect, int cornerRadius)
{GraphicsPath roundedRect = new GraphicsPath();roundedRect.AddArc(rect.X, rect.Y, cornerRadius * 2, cornerRadius * 2, 180, 90);roundedRect.AddLine(rect.X + cornerRadius, rect.Y, rect.Right - cornerRadius * 2, rect.Y);roundedRect.AddArc(rect.X + rect.Width - cornerRadius * 2, rect.Y, cornerRadius * 2, cornerRadius * 2, 270, 90);roundedRect.AddLine(rect.Right, rect.Y + cornerRadius * 2, rect.Right, rect.Y + rect.Height - cornerRadius * 2);roundedRect.AddArc(rect.X + rect.Width - cornerRadius * 2, rect.Y + rect.Height - cornerRadius * 2, cornerRadius * 2, cornerRadius * 2, 0, 90);roundedRect.AddLine(rect.Right - cornerRadius * 2, rect.Bottom, rect.X + cornerRadius * 2, rect.Bottom);roundedRect.AddArc(rect.X, rect.Bottom - cornerRadius * 2, cornerRadius * 2, cornerRadius * 2, 90, 90);roundedRect.AddLine(rect.X, rect.Bottom - cornerRadius * 2, rect.X, rect.Y + cornerRadius * 2);roundedRect.CloseFigure();return roundedRect;
}

2、绘制阴影

internal void DrawShadow()
{Bitmap bitmap = null;Graphics g = null;try{bitmap = new Bitmap(Width, Height);g = Graphics.FromImage(bitmap);g.SmoothingMode = SmoothingMode.AntiAlias;Color c = Color.FromArgb(0, 0, 0, 0);Pen p = new Pen(c, 3);for (int i = 0; i < Main.ShadowWidth; i++){p.Color = Color.FromArgb((255 / 10 / Main.ShadowWidth) * i, c);DrawRoundRectangle(g, p, new Rectangle(i, i, Width - (2 * i) - 1, Height - (2 * i) - 1), Main.ShadowWidth - i);}SetBits(bitmap);}catch { }finally{g?.Dispose();bitmap?.Dispose();}
}

3、绘制半透明bitmap到窗口上

protected override CreateParams CreateParams
{get{CreateParams cParms = base.CreateParams;cParms.ExStyle |= 0x00080000; // WS_EX_LAYEREDreturn cParms;}
}
public void SetBits(Bitmap bitmap)
{if (!Image.IsCanonicalPixelFormat(bitmap.PixelFormat) || !Image.IsAlphaPixelFormat(bitmap.PixelFormat))throw new ApplicationException("图片必须是32位带Alhpa通道的图片。");IntPtr oldBits = IntPtr.Zero;IntPtr screenDC = FormStyleAPI.GetDC(IntPtr.Zero);IntPtr hBitmap = IntPtr.Zero;IntPtr memDc = FormStyleAPI.CreateCompatibleDC(screenDC);try{FormStyleAPI.Point topLoc = new FormStyleAPI.Point(Left, Top);FormStyleAPI.Size bitMapSize = new FormStyleAPI.Size(Width, Height);FormStyleAPI.BLENDFUNCTION blendFunc = new FormStyleAPI.BLENDFUNCTION();FormStyleAPI.Point srcLoc = new FormStyleAPI.Point(0, 0);hBitmap = bitmap.GetHbitmap(Color.FromArgb(0));oldBits = FormStyleAPI.SelectObject(memDc, hBitmap);blendFunc.BlendOp = FormStyleAPI.AC_SRC_OVER;blendFunc.SourceConstantAlpha = Byte.Parse(((int)(Main.Opacity * 255)).ToString());blendFunc.AlphaFormat = FormStyleAPI.AC_SRC_ALPHA;blendFunc.BlendFlags = 0;FormStyleAPI.UpdateLayeredWindow(Handle, screenDC, ref topLoc, ref bitMapSize, memDc, ref srcLoc, 0, ref blendFunc, FormStyleAPI.ULW_ALPHA);}finally{if (hBitmap != IntPtr.Zero){FormStyleAPI.SelectObject(memDc, oldBits);FormStyleAPI.DeleteObject(hBitmap);}FormStyleAPI.ReleaseDC(IntPtr.Zero, screenDC);FormStyleAPI.DeleteDC(memDc);}
}

四、最终方案的效果和源码

源码下载