用C#设计一个复数类，重载运算符，实现复数的加法和乘法运算。并实现复数到浮点数的强制类型转换。 设计Main()函数，使用此复数类。

下面是一个用C#设计的复数类，包括重载运算符，实现复数的加法和乘法运算，并实现复数到浮点数的强制类型转换：

using System;

namespace ComplexNumber
{
    class Complex
    {
        private double real;
        private double imag;

        public Complex(double real, double imag)
        {
            this.real = real;
            this.imag = imag;
        }

        public static Complex operator +(Complex a, Complex b)
        {
            return new Complex(a.real + b.real, a.imag + b.imag);
        }

        public static Complex operator *(Complex a, Complex b)
        {
            return new Complex(a.real * b.real - a.imag * b.imag, a.real * b.imag + a.imag * b.real);
        }

        public static explicit operator double(Complex a)
        {
            return Math.Sqrt(a.real * a.real + a.imag * a.imag);
        }

        public override string ToString()
        {
            return string.Format("{0} + {1}i", real, imag);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Complex a = new Complex(1, 2);
            Complex b = new Complex(3, 4);

            Complex c = a + b;
            Console.WriteLine("a + b = {0}", c);

            Complex d = a * b;
            Console.WriteLine("a * b = {0}", d);

            double e = (double)a;
            Console.WriteLine("a = {0}, |a| = {1}", a, e);

            Console.ReadKey();
        }
    }
}

在Main()函数中，我们创建了两个复数对象a和b，然后分别进行加法运算和乘法运算，并输出结果。我们还将复数a强制类型转换为浮点数，并输出其模长。