Hi Sattar,
Thank you for your interest in Syncfusion Products.
How we can create Flow Chart Controls(Decision,Circle, etc..) You can create a decision box by rotating a rectangle to 45 degree and circle by using ellipse.
//Decision Box
Syncfusion.Windows.Forms.Diagram.Rectangle decision = new Syncfusion.Windows.Forms.Diagram.Rectangle(100, 200, 60, 60);
//Rotate to make as decision box
decision.RotationAngle = 45;
decision.Visible = true;
//specify the height and width as same to get circle
Syncfusion.Windows.Forms.Diagram.Ellipse ellipse = new Ellipse(40, 50, 60, 60);
ellipse .FillStyle.Color = Color.Black;
Please refer the sample in the below link which illustrates the above:
http://websamples.syncfusion.com/samples/Diagram.Web/5.2.0.25/69959/main.htmPlease try this and let me know if this helps.
Thanks,
Melba