Já viu a propriedade no site da Microsoft?
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Propriedade Border.CornerRadius
https://msdn.microsoft.com/pt-br/library/system.windows.controls.border.cornerradius(v=vs.110).aspx
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Dim myBorder1 As New Border()
myBorder1.BorderBrush = Brushes.SlateBlue
myBorder1.BorderThickness = New Thickness(5, 10, 15, 20)
myBorder1.Background = Brushes.AliceBlue
myBorder1.Padding = New Thickness(5)
myBorder1.CornerRadius = New CornerRadius(15)
Veja também:
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Creating Custom TabControls in Visual Studio 2015 vb
Edwin Vincent
Publicado em 14 de jul de 2016
Copy & Paste AREA 1 and AREA 2 into you code from:
(Source Code also available here)
http://pastebin.com/sfH94KBP
Area 1: line 175 to line 192
Area 2: line 222 to line 254
https://youtu.be/vo4ygDRiy1k
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[]'s
Fabio I.