如何在C# .NET中实现方法依赖注入并解决日志参数缺失问题

我尝试实现一个写入文件的日志记录器，日志文件路径存储在appSettings.json中，已通过模型类关联该配置。以下是我的LogWriter.cs代码：

public class LogWriter
{
    public void WriteLog(string message, IOptions<AppSettings> appSettings)
    {
        AppSettings _appSettings = appSettings.Value;
        using (StreamWriter writer = new StreamWriter(_appSettings.LogFilePath))
        {
            writer.Write("[" + DateTime.Now.ToString("MM/dd/yyyy HH:mm:ss") +"] " + message);
        }
    }
}

我希望通过WriteLog方法注入appSettings依赖，无需实例化LogWriter类即可调用该方法，以下是program.cs的依赖配置代码：

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);

var appSettings = new AppSettings();
builder.Configuration.GetSection("AppSettings").Bind(appSettings);

//Add services to the container.
builder.Services.Configure<AppSettings>(builder.Configuration.GetSection("AppSettings"));
builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

builder.Services.AddScoped<IMessageService, MessageService>();
builder.Services.AddScoped<LogWriter>();// DI for LogWriter

builder.Services.AddDbContext<MailContext>(
    o => o.UseNpgsql(appSettings.ConnectionStrings.MailDb)
    );

var app = builder.Build();

app.UseSwagger();
app.UseSwaggerUI();

app.UseAuthorization();
app.MapControllers();

app.Run();

但当我在控制器中以LogWriter.WriteLog("Construktor wird ausgeführt")方式调用时，提示缺少一个参数，控制器代码如下：

[Route("[controller]")]
[ApiController]
public class MessageController : ControllerBase
{
    private readonly IMessageService _mailService;

    public MessageController(IMessageService mailServiceCustom )
    {
        _mailService = mailServiceCustom;
        LogWriter.WriteLog("Construktor wird ausgeführt");

    }
// ...
}

方案一：遵循.NET依赖注入最佳实践（推荐） #

这种方式通过构造函数注入依赖，符合DI设计原则，便于测试和维护：

1. 重构LogWriter，将配置依赖移至构造函数
   把IOptions<AppSettings>通过构造函数注入，同时修复日志写入时的覆盖问题（添加append: true参数，并用WriteLine实现换行）：

   public class LogWriter
   {
       private readonly AppSettings _appSettings;
   
       // 构造函数注入配置
       public LogWriter(IOptions<AppSettings> appSettings)
       {
           _appSettings = appSettings.Value;
       }
   
       public void WriteLog(string message)
       {
           // 追加写入日志，避免覆盖原有内容
           using (StreamWriter writer = new StreamWriter(_appSettings.LogFilePath, append: true))
           {
               writer.WriteLine($"[{DateTime.Now:MM/dd/yyyy HH:mm:ss}] {message}");
           }
       }
   }
   

2. 在控制器中注入LogWriter实例
   修改控制器构造函数，注入LogWriter，然后调用实例方法：

   [Route("[controller]")]
   [ApiController]
   public class MessageController : ControllerBase
   {
       private readonly IMessageService _mailService;
       private readonly LogWriter _logWriter;
   
       // 构造函数注入LogWriter
       public MessageController(IMessageService mailServiceCustom, LogWriter logWriter)
       {
           _mailService = mailServiceCustom;
           _logWriter.WriteLog("Construktor wird ausgeführt");
       }
   
       // 其他控制器方法...
   }
   

3. 保持现有Program.cs配置不变
   你已经添加了builder.Services.AddScoped<LogWriter>();，这部分配置无需修改，DI容器会自动处理依赖注入。

方案二：静态调用方式（不推荐） #

如果一定要无需实例化即可调用，可以将LogWriter改为静态类，并在程序启动时初始化配置：

1. 修改LogWriter为静态类

   public static class LogWriter
   {
       private static AppSettings _appSettings;
   
       // 初始化配置，在程序启动时调用
       public static void Initialize(IOptions<AppSettings> appSettings)
       {
           _appSettings = appSettings.Value;
       }
   
       public static void WriteLog(string message)
       {
           using (StreamWriter writer = new StreamWriter(_appSettings.LogFilePath, append: true))
           {
               writer.WriteLine($"[{DateTime.Now:MM/dd/yyyy HH:mm:ss}] {message}");
           }
       }
   }
   

2. 在Program.cs中初始化静态类

   var app = builder.Build();
   
   // 获取配置服务并初始化静态LogWriter
   var appSettings = app.Services.GetRequiredService<IOptions<AppSettings>>();
   LogWriter.Initialize(appSettings);
   
   // 后续中间件配置不变
   app.UseSwagger();
   app.UseSwaggerUI();
   // ...
   

3. 控制器中静态调用
   此时可以直接调用LogWriter.WriteLog("Construktor wird ausgeführt");，无需注入实例。

注意：静态类方式耦合度高，难以进行单元测试，不推荐在生产环境使用。

内容的提问来源于stack exchange，提问作者Razielruss