c#三层结构写小程序后端登录接口

好的，以下是一个简单的C#三层结构写的小程序后端登录接口的示例：

1. 数据层（Data Access Layer）

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace YourNamespace.Data
{
    public class UserDataAccess
    {
        private readonly string connectionString;

        public UserDataAccess(string connectionString)
        {
            this.connectionString = connectionString;
        }

        public bool ValidateUser(string username, string password)
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                string query = "SELECT COUNT(*) FROM Users WHERE Username = @Username AND Password = @Password";
                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    command.Parameters.AddWithValue("@Username", username);
                    command.Parameters.AddWithValue("@Password", password);
                    connection.Open();
                    int count = (int) command.ExecuteScalar();
                    return count > 0;
                }
            }
        }
    }
}

2. 业务逻辑层（Business Logic Layer）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using YourNamespace.Data;

namespace YourNamespace.BusinessLogic
{
    public class UserLogic
    {
        private readonly UserDataAccess userDataAccess;

        public UserLogic(UserDataAccess userDataAccess)
        {
            this.userDataAccess = userDataAccess;
        }

        public bool ValidateUser(string username, string password)
        {
            return userDataAccess.ValidateUser(username, password);
        }
    }
}

3. 表现层（Presentation Layer）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using YourNamespace.BusinessLogic;
using YourNamespace.Data;

namespace YourNamespace.Presentation
{
    class Program
    {
        static void Main(string[] args)
        {
            string connectionString = "your connection string";
            UserDataAccess userDataAccess = new UserDataAccess(connectionString);
            UserLogic userLogic = new UserLogic(userDataAccess);

            Console.WriteLine("Please enter your username:");
            string username = Console.ReadLine();

            Console.WriteLine("Please enter your password:");
            string password = Console.ReadLine();

            if (userLogic.ValidateUser(username, password))
            {
                Console.WriteLine("Login successful!");
            }
            else
            {
                Console.WriteLine("Invalid username or password.");
            }

            Console.ReadLine();
        }
    }
}

注意，以上示例只是一个简单的演示，实际应用可能需要更严格的安全性和错误处理。