// 重构前：职责混乱
public class UserService  
{
    public void S*eUser(User user)
    {
        // 保存用户
        File.WriteAllText("users.txt", user.Name);
        // 发送通知
        Console.WriteLine("User s*ed: " + user.Name);
    }
}
<p>// 重构后：职责分离
public class UserRepository { public void S*e(User user) => File.WriteAllText("users.txt", user.Name); }
public class NotificationService { public void Notify(string message) => Console.WriteLine(message); }</p><p>public class UserService
{
private readonly UserRepository _repo = new();
private readonly NotificationService _notifier = new();</p><pre class='brush:php;toolbar:false;'>public void S*eUser(User user)
{
    _repo.S*e(user);
    _notifier.Notify("User s*ed: " + user.Name);
}

}

// 定义折扣策略接口
public interface IDiscountStrategy
{
    decimal ApplyDiscount(decimal price);
}
<p>public class RegularDiscount : IDiscountStrategy { public decimal ApplyDiscount(decimal price) => price <em> 0.9m; }
public class VIPDiscount : IDiscountStrategy { public decimal ApplyDiscount(decimal price) => price </em> 0.7m; }</p><p>public class OrderProcessor
{
private readonly IDiscountStrategy _strategy;</p><pre class='brush:php;toolbar:false;'>public OrderProcessor(IDiscountStrategy strategy) => _strategy = strategy;

public decimal CalculatePrice(decimal price) => _strategy.ApplyDiscount(price);

}

public interface IPrinter { void Print(); }
public interface IScanner { void Scan(); }
<p>public class SimplePrinter : IPrinter { public void Print() => Console.WriteLine("Printing..."); }
public class MultiFunctionDevice : IPrinter, IScanner
{
public void Print() => Console.WriteLine("Multifunction print");
public void Scan() => Console.WriteLine("Scanning...");
}
</font></p>

这样客户端只引用所需接口，避免强制实现无用方法。

依赖倒置原则（DIP）

高层模块不应依赖低层模块，二者都应依赖抽象；抽象不应依赖细节，细节应依赖抽象。

示例：

public interface IDataAccess { List<User> GetUsers(); }
public class SqlDataAccess : IDataAccess { /* 实现 */ }
public class UserService
{
    private readonly IDataAccess _dataAccess;
    public UserService(IDataAccess dataAccess) => _dataAccess = dataAccess;
}

UserService不关心具体数据来源，便于替换为内存、文件或其他数据库实现。

基本上就这些。在C#项目中持续应用SOLID原则，配合单元测试、代码审查和重构工具（如ReSharper），能有效提升代码质量。注意：SOLID不是教条，需结合实际场景权衡使用，避免过度设计。