WPF rectangle - round just top corners
Set the RadiusX and RadiusY properties on the rectangle, this will give it rounded corners
Good example how its possible to do OnRender with DrawingContext:
/// <summary>
/// Draws a rounded rectangle with four individual corner radius
/// </summary>
public static void DrawRoundedRectangle(this DrawingContext dc, Brush brush,
Pen pen, Rect rect, CornerRadius cornerRadius)
{
var geometry = new StreamGeometry();
using (var context = geometry.Open())
{
bool isStroked = pen != null;
const bool isSmoothJoin = true;
context.BeginFigure(rect.TopLeft + new Vector(0, cornerRadius.TopLeft), brush != null, true);
context.ArcTo(new Point(rect.TopLeft.X + cornerRadius.TopLeft, rect.TopLeft.Y),
new Size(cornerRadius.TopLeft, cornerRadius.TopLeft),
90, false, SweepDirection.Clockwise, isStroked, isSmoothJoin);
context.LineTo(rect.TopRight - new Vector(cornerRadius.TopRight, 0), isStroked, isSmoothJoin);
context.ArcTo(new Point(rect.TopRight.X, rect.TopRight.Y + cornerRadius.TopRight),
new Size(cornerRadius.TopRight, cornerRadius.TopRight),
90, false, SweepDirection.Clockwise, isStroked, isSmoothJoin);
context.LineTo(rect.BottomRight - new Vector(0, cornerRadius.BottomRight), isStroked, isSmoothJoin);
context.ArcTo(new Point(rect.BottomRight.X - cornerRadius.BottomRight, rect.BottomRight.Y),
new Size(cornerRadius.BottomRight, cornerRadius.BottomRight),
90, false, SweepDirection.Clockwise, isStroked, isSmoothJoin);
context.LineTo(rect.BottomLeft + new Vector(cornerRadius.BottomLeft, 0), isStroked, isSmoothJoin);
context.ArcTo(new Point(rect.BottomLeft.X, rect.BottomLeft.Y - cornerRadius.BottomLeft),
new Size(cornerRadius.BottomLeft, cornerRadius.BottomLeft),
90, false, SweepDirection.Clockwise, isStroked, isSmoothJoin);
context.Close();
}
dc.DrawGeometry(brush, pen, geometry);
}
Information from: http://wpftutorial.net/DrawRoundedRectangle.html
The problem you've got is that the rectangle is "overflowing" the rounded corners of your border.
A rectangle can't have individually rounded corners, so if you just put the background colour on the border and remove the rectangle:
<Border BorderThickness="1" Grid.Row="0" Grid.ColumnSpan="2"
CornerRadius="50,50,0,0" BorderBrush="Black" Background="#FF5A9AE0">
</Border>
You'll get your desired effect.