مساعدة في تصحيح خطأ

الخطأ

A switch expression or case label must be a bool, char, string, integral, enum, or corresponding nullable type

الحرف د الذي تحته خط هو الخطأ.

الكود

 System.Console.WriteLine("Enter //a// value: ");
            double a=System.Convert.ToDouble(System.Console.ReadLine());
            System.Console.WriteLine("Enter //b// value: ");
            double b = System.Convert.ToDouble(System.Console.ReadLine());
            System.Console.WriteLine("Enter //c// value: ");
            double c = System.Convert.ToDouble(System.Console.ReadLine());
            double D = System.Math.Pow(b, 2) - 4 * a * c;
            switch (D) 
                {
                    case D < 0:
                        System.Console.WriteLine("Sorry, not exist solution.");
                        break;
                    case D = 0:
                        System.Console.WriteLine(-b/(2*a));
                        break;
                    case D > 0:
                        System.Console.WriteLine((-b+System.Math.Sqrt(D))/(2*a)+"  "+(-b-System.Math.Sqrt(D))/(2*a));
                        break;
                }
            System.Console.ReadLine();

شكرا.