Public Class Form1
    '*********************************************************************   
    '作者：章鱼哥，QQ：3107073263 群：309816713       
    '如有疑问或好的建议请联系我，大家一起进步     
    '*********************************************************************     
    '绘制圆角矩形函数
    Private Function GetRoundedRectPath(ByVal rect As Rectangle, ByVal radius As Integer) As System.Drawing.Drawing2D.GraphicsPath
        rect.Offset(-1, -1)
        Dim RoundRect As New Rectangle(rect.Location, New Size(radius - 1, radius - 1))

        Dim path As New System.Drawing.Drawing2D.GraphicsPath
        path.AddArc(RoundRect, 180, 90)     '左上角
        RoundRect.X = rect.Right - radius   '右上角
        path.AddArc(RoundRect, 270, 90)
        RoundRect.Y = rect.Bottom - radius  '右下角
        path.AddArc(RoundRect, 0, 90)
        RoundRect.X = rect.Left             '左下角
        path.AddArc(RoundRect, 90, 90)
        path.CloseFigure()
        Return path
    End Function
    '绘制矩形
    Private Sub DrawingRect()
        Dim g As Graphics = Me.CreateGraphics '定义一个画布
        Dim Pen As New Pen(Brushes.DarkRed, 2) '定义一个画笔
        Dim Hei As Integer = Me.Height
        Dim Wid As Integer = Me.Width
        '矩形的位置和长宽随着窗体的变化而改变
        Dim Rec As New Rectangle(Int(Wid / 5), Int(Hei / 5), Int(Wid / 2), Int(Hei / 2))
        '  g.DrawRectangle(Pen, Rec)
        '清楚现有的矩形
        g.Clear(Me.BackColor)
        g.DrawPath(Pen, GetRoundedRectPath(Rec, 30))
    End Sub

    Private Sub Form1_Paint(ByVal sender As System.Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint
        DrawingRect()
    End Sub

    Private Sub Form1_SizeChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.SizeChanged
        Me.Invalidate() '此函数可引发Paint事件
    End Sub
End Class

章鱼哥出品—VB.NET 如何绘制圆角矩形，并适应窗体大小