c#中switch语句怎么用，不能再case中赋值吗，为什么我这个最后局部变量f未赋值？

public static double tuoqiu()
        {
            Console.WriteLine("请选择椭球类型：");
            Console.WriteLine("如果为克拉索夫斯基椭球请输入1");
            Console.WriteLine("如果为IAG-75椭球请输入2");
            Console.WriteLine("如果为WGS-84椭球请输入3");
            Console.WriteLine("如果为CGCS2000椭球请输入4");
            string tuoqiu = Console.ReadLine();
            int temp = int.Parse(tuoqiu);
            double f;
            switch (temp)
            {
                case 1: f = 1 / 298.3; break;
                case 2: f = 1 / 298.257; break;
                case 3: f = 1 / 298.257223563; break;
                case 4: f = 1 / 298.257222101; break;
            }
            return f;
        }

double f=0; 初始化一下