1.VS2019创建C#主程序

2.编译主程序

debug目录下生成exe：

3.添加类库工程，并添加计算逻辑

添加计算逻辑：

namespace Calculate
{
    public class CalculateAssistant
    {
        // +
        public double Sum(double a, double b)
        {
            return a + b;
        }
        public Int64 Sum(Int64 a, Int64 b)
        {
            return a + b;
        }
        // -
        public double Sub(double a, double b)
        {
            return a - b;
        }
        public Int64 Sub(Int64 a, Int64 b)
        {
            return a - b;
        }
        // *
        public double Multiple(double a, double b)
        {
            return a * b;
        }
        public Int64 Multiple(Int64 a, Int64 b)
        {
            return a * b;
        }
        // /
        public double Divide(double a, double b)
        {
            return a / b;
        }
        public Int64 Divide(Int64 a, Int64 b)
        {
            return a / b;
        }
    }
}

4.给主程序添加引用项

5.重新编译主程序

先生成依赖项目，再生成主程序：

debug目录增加了dll文件：

6.主程序添加测试逻辑

using Calculate;

namespace Main
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Add_Click(object sender, EventArgs e)
        {
            Calculate.CalculateAssistant calc = new CalculateAssistant();
            double result = calc.Sum(double.Parse(textBox1.Text),double.Parse(textBox2.Text));
            textBox3.Text = result.ToString();            
        }

        private void Sub_Click(object sender, EventArgs e)
        {
            Calculate.CalculateAssistant calc = new CalculateAssistant();
            double result = calc.Sub(double.Parse(textBox1.Text), double.Parse(textBox2.Text));
            textBox3.Text = result.ToString();
        }

        private void Multi_Click(object sender, EventArgs e)
        {
            Calculate.CalculateAssistant calc = new CalculateAssistant();
            double result = calc.Multiple(double.Parse(textBox1.Text), double.Parse(textBox2.Text));
            textBox3.Text = result.ToString();
        }

        private void Divide_Click(object sender, EventArgs e)
        {
            Calculate.CalculateAssistant calc = new CalculateAssistant();
            double result = calc.Divide(double.Parse(textBox1.Text), double.Parse(textBox2.Text));
            textBox3.Text = result.ToString();
        }
    }
}