C 源码：

#include <stdio.h>
#pragma comment(lib, "DLL_demo.lib")

extern double add(double, double);
extern double sub(double, double);
extern double mul(double, double);
extern double div(double, double);

int main(void)
{
    double a, b;
    char op, c;
    puts("Please input a formula and get the result. (enter q to quit)\n");
    while (1)
    {
        printf("Enter: ");
        if (scanf("%lf %c %lf", &a, &op, &b) == 3)
            switch (op)
            {
                case '+':printf("%.2lf + %.2lf = %.2lf\n", a, b, add(a, b)); break;
                case '-':printf("%.2lf - %.2lf = %.2lf\n", a, b, sub(a, b)); break;
                case '*':printf("%.2lf * %.2lf = %.2lf\n", a, b, mul(a, b)); break;
                case '/':printf("%.2lf / %.2lf = %.2lf\n", a, b, div(a, b)); break;
                default :printf("Enter error!\n");
            }
        else
        {
            if ((c = getchar()) == 'q')
                break;
            puts("Enter error!");
            while (getchar() != '\n')
                continue;
        }
    }
    puts("Bye~");
    getchar();
    getchar();

    return 0;
}

DLL 源码：

_declspec(dllexport) double add(double a, double b)
{
    return a + b;
}

_declspec(dllexport) double sub(double a, double b)
{
    return a - b;
}

_declspec(dllexport) double mul(double a, double b)
{
    return a * b;
}

_declspec(dllexport) double div(double a, double b)
{
    return a / b;
}