关于获得线程返回值的问题

delegate double CalculateMethod(double Diameter);
        static CalculateMethod calcMethod;
        double result = 0;        static void Main(string[] args)
        {
            calcMethod = new CalculateMethod(Calculate);
            calcMethod.BeginInvoke(5, new AsyncCallback(TaskFinished), null);
        }
        /// 
        ///线程调用的函数 
        /// 
        public static double Calculate(double Diameter)
        {
            return Diameter * Math.PI;
        }
        /// 
        ///线程完成之后回调的函数 
        /// 
        public static void TaskFinished(IAsyncResult result)
        {
            result = calcMethod.EndInvoke(result);
        }编译不过 出现 错误 CS0029: 无法将类型“double”隐式转换为“System.IAsyncResult”