C# 关于”程序不包含适合于入口点的静态“Main”方法“的问题

C#练习题“定义完整的complexNumber类，在其中重载加减乘除等基本算术操作符，相等与不等操作符（比较复数的实部和虚部是否均相等），以及大小比较操作符（比较复数模的大小）。”

解决该问题，我所写出的代码为：

namespace ConsoleApp1
{
#pragma warning disable CS0660 // 类型定义运算符 == 或运算符 !=，但不重写 Object.Equals(object o)
    public class ComplexNumber
#pragma warning restore CS0660 // 类型定义运算符 == 或运算符 !=，但不重写 Object.Equals(object o)
    {
        private double x, y;
        public double X
        {
            get { return x; }
            set { x = value; }
        }

        public double Y //虚数部分
        {
            get { return y; }
            set { y = value; }
        }
        public ComplexNumber(double x, double y)
        {
            this.x = x;
            this.y = y;
        }

        public static ComplexNumber operator +(ComplexNumber c1, ComplexNumber c2)
        {
            return new ComplexNumber(c1.x + c2.x, c1.y + c2.y);
        }
        public static ComplexNumber operator -(ComplexNumber c1, ComplexNumber c2)
        {
            return new ComplexNumber(c1.x - c2.x, c1.y - c2.y);
        }
        public static bool operator ==(ComplexNumber c1, ComplexNumber c2)
        {
            return c1.x == c2.x && c1.y == c2.y;
        }
        public static bool operator !=(ComplexNumber c1, ComplexNumber c2)
        {
            return c1.x != c2.x || c1.y != c2.y;
        }
        public static bool operator >(ComplexNumber c1, ComplexNumber c2)
        {
            double a = c1.x * c1.x + c1.y * c1.y;
            double b = c2.x * c2.x + c2.y * c2.y;
            return a > b;
        }
        public static bool operator >=(ComplexNumber c1, ComplexNumber c2)
        {
            double a = c1.x * c1.x + c1.y * c1.y;
            double b = c2.x * c2.x + c2.y * c2.y;
            return a == b || a > b;
        }
        public static bool operator <(ComplexNumber c1, ComplexNumber c2)
        {
            return !(c1 >= c2);
        }
        public static bool operator <=(ComplexNumber c1, ComplexNumber c2)
        {
            return !(c1 > c2);
        }

        public override bool Equals(object obj)
        {
            return base.Equals(obj);
        }

        public override int GetHashCode()
        {
            return base.GetHashCode();
        }

        public override string ToString()
        {
            return base.ToString();
        }
    }
}

在修改完一些小错误后，代码逻辑基本正确，但是最后一个问题仍然无法解决

在网络上查找到更改运行环境到”类库“但是会出现新问题

所以这个缺少”Main“的错误到底怎么解决，是代码缺少东西吗
求各位大神帮忙解决，感激不尽~~~

你只有一个类，没有程序入口。
你可以定义一个
static void Main(string[] args)
{
//在这里可以调用
ComplexNumber cn = new ComplexNumber(1, 2);
...
}