WPF using WindowsXamlHost can set it from quadrilateral to triangle ？

Question

when WPF use WindowsXamlHost it default quadrilateral and background is white
can we make it triangle?

Answer 1

A way to change its shape is with SetWindowRgn

A test :

IntPtr hWndHost = windowsXamlHostControl.Handle;  
POINT[] pt;  
pt = new POINT[5];        
pt[0].x = (int)windowsXamlHostControl.Width/2;  
pt[0].y = 0;  
pt[1].x = (int)windowsXamlHostControl.Width;  
pt[1].y = (int)windowsXamlHostControl.Height / 2;  
pt[2].x = (int)windowsXamlHostControl.Width / 2;  
pt[2].y = (int)windowsXamlHostControl.Height;  
pt[3].x = 0;  
pt[3].y = (int)windowsXamlHostControl.Height / 2;  
pt[4].x = (int)windowsXamlHostControl.Width / 2;  
pt[4].y = 0;  
IntPtr hRgn = CreatePolygonRgn(ref pt[0], 10, 1);  
SetWindowRgn(hWndHost, hRgn, true);  

Declarations :

[DllImport("user32.dll", SetLastError = true)]  
private static extern int SetWindowRgn(IntPtr hWnd, IntPtr hRgn, bool bRedraw);  
  
[DllImport("gdi32.dll", SetLastError = true)]  
private static extern IntPtr CreateRoundRectRgn(int x1, int y1, int x2, int y2, int cx, int cy);  
  
[DllImport("gdi32.dll", SetLastError = true)]  
private static extern IntPtr CreatePolygonRgn(ref POINT lpPoint, int nCount, int nPolyFillMode);  
  
[StructLayout(LayoutKind.Sequential)]  
public struct POINT  
{  
    public int x;  
    public int y;  
    public POINT(int X, int Y)  
    {  
        this.x = X;  
        this.y = Y;  
    }  
}