MediatR：从青铜到王者，解锁.NET消息处理终极奥义！

云中小生

发布于 2025-08-24 08:55:44

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🌟 开篇：初识 MediatR

在.NET 的修仙世界中，MediatR 就像是一本神奇的"传音入密"秘籍，它能让你的代码各司其职，却又心意相通。今天，就让我们一起来修炼这本秘籍，掌握.NET 中的中介者模式精髓！ MediatR 是一个简单的中介者模式实现，它通过解耦消息发送者和接收者来简化应用程序中的进程内通信。简单来说，它就像是一个邮局，负责把你的"信件"(消息)准确无误地投递给正确的"收件人"(处理器)。


// 安装MediatR
dotnet add package MediatR

🏗️ 第一重境界：基础搭建

1. 配置 MediatR

首先，我们需要在 Startup.cs 或 Program.cs 中注册 MediatR 服务：


// .NET 6+ 的Program.cs
builder.Services.AddMediatR(cfg =>
    cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly()));

2. 创建第一个消息和处理器

让我们从一个简单的"Hello World"开始：


// 定义消息
public class HelloWorldQuery:IRequest<string>
{
    publicstring Name {get;set;}
}

// 定义处理器
public class HelloWorldHandler

    :IRequestHandler<HelloWorldQuery,string>
{
    public Task<string>  Handle(

        HelloWorldQuery request,

        CancellationToken cancellationToken)
    {
        return Task.FromResult($"Hello, {request.Name}!");
    }
}

// 使用示例
var response =await mediator.Send(new HelloWorldQuery{ Name ="修仙者"});
Console.WriteLine(response);// 输出: Hello, 修仙者!

🧩 第二重境界：命令与查询

MediatR 支持两种主要模式：命令(Command)和查询(Query)，对应 CQRS 模式。

1. 命令模式示例


// 创建用户命令
public class CreateUserCommand:IRequest<int>
{
    public string Username {get;set;}
    public string Email {get;set;}
}

public class CreateUserHandler

  :IRequestHandler<CreateUserCommand,int>
{
    privatereadonlyApplicationDbContext _context;

    public CreateUserHandler(ApplicationDbContext context)
    {
        _context = context;
    }

    public async Task<int>Handle(

        CreateUserCommand request,

        CancellationToken cancellationToken)
    {
        var user =new User
        {
            Username = request.Username,
            Email = request.Email
        };

        _context.Users.Add(user);
        await _context.SaveChangesAsync(cancellationToken);

        return user.Id;
    }
}

2. 查询模式示例


// 获取用户详情查询
public class GetUserByIdQuery:IRequest<UserDto>
{
    publicint UserId {get;set;}
}

public class GetUserByIdHandler

    :IRequestHandler<GetUserByIdQuery, UserDto>
{
    privatereadonlyApplicationDbContext _context;

    public GetUserByIdHandler(ApplicationDbContext context)
    {
        _context = context;
    }


public asyncTask<UserDto>Handle(GetUserByIdQuery request,CancellationToken cancellationToken)
{
var user =await _context.Users
.Where(u => u.Id == request.UserId)
.Select(u =>new UserDto
{
                Id = u.Id,
                Username = u.Username,
                Email = u.Email
})
.FirstOrDefaultAsync(cancellationToken);

return user ??throw new NotFoundException("User not found");
}
}

🔗 第三重境界：通知与事件

MediatR 还支持发布/订阅模式，通过 INotification 接口实现。

1. 定义通知事件


publicclassUserCreatedNotification:INotification
{
    public int UserId {get;set;}
    public string Username {get;set;}
    public DateTime CreatedAt {get;set;}
}

2. 创建多个处理器


// 发送欢迎邮件处理器
public class SendWelcomeEmailHandler:INotificationHandler<UserCreatedNotification>
{
    public async Task Handle(UserCreatedNotification notification,CancellationToken cancellationToken)
{
    // 模拟发送邮件
    Console.WriteLine($"发送欢迎邮件给 {notification.Username}");
    await Task.Delay(1000);
}
}

// 记录用户创建日志处理器
public class LogUserCreatedHandler:INotificationHandler<UserCreatedNotification>
{
private readonly ILogger<LogUserCreatedHandler> _logger;

public LogUserCreatedHandler(ILogger<LogUserCreatedHandler> logger)
{
        _logger = logger;
}

public Task Handle(UserCreatedNotification notification,CancellationToken cancellationToken)
{
        _logger.LogInformation("新用户创建: {UserId}, 用户名: {Username}",
            notification.UserId, notification.Username);
return Task.CompletedTask;
}
}

3. 发布通知


await mediator.Publish(new UserCreatedNotification
{
    UserId = newUserId,
    Username = command.Username,
    CreatedAt = DateTime.UtcNow
});

🛡️ 第四重境界：管道行为

MediatR 的管道行为(Pipeline Behaviors)类似于 ASP.NET Core 的中间件，可以在处理请求前后执行逻辑

1. 创建日志行为


public class LoggingBehavior<TRequest, TResponse>:IPipelineBehavior<TRequest, TResponse>
where TRequest:IRequest<TResponse>
{
private readonly ILogger<LoggingBehavior<TRequest, TResponse>> _logger;

public LoggingBehavior(ILogger<LoggingBehavior<TRequest, TResponse>> logger)
{
        _logger = logger;
}

public async Task<TResponse>Handle(TRequest request,RequestHandlerDelegate<TResponse> next,CancellationToken cancellationToken)
{
        _logger.LogInformation("处理请求 {RequestName} {@Request}",
typeof(TRequest).Name, request);

try
{
var response = await next();
            _logger.LogInformation("请求 {RequestName} 处理成功",typeof(TRequest).Name);
return response;
}
catch(Exception ex)
{
            _logger.LogError(ex,"请求 {RequestName} 处理出错",typeof(TRequest).Name);
throw;
}
}
}

2. 创建验证行为


public class ValidationBehavior<TRequest, TResponse>:IPipelineBehavior<TRequest, TResponse>
where TRequest:IRequest<TResponse>
{
private readonly IEnumerable<IValidator<TRequest>> _validators;

public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
{
        _validators = validators;
}

public async Task<TResponse>Handle(TRequest request,RequestHandlerDelegate<TResponse> next,CancellationToken cancellationToken)
{
if(_validators.Any())
{
var context =new ValidationContext<TRequest>(request);

var validationResults =await Task.WhenAll(
                _validators.Select(v => v.ValidateAsync(context, cancellationToken)));

var failures = validationResults
.SelectMany(r => r.Errors)
.Where(f => f !=null)
.ToList();

if(failures.Count !=0)
throw new ValidationException(failures);
}

return await next();
}
}

3. 注册行为


builder.Services.AddTransient(typeof(IPipelineBehavior<,>),typeof(LoggingBehavior<,>));
builder.Services.AddTransient(typeof(IPipelineBehavior<,>),typeof(ValidationBehavior<,>));

🎯 第五重境界：高级技巧

1. 使用 FluentValidation 进行验证


public class CreateUserCommandValidator:AbstractValidator<CreateUserCommand>
{
public CreateUserCommandValidator()
{
    RuleFor(x => x.Username)
    .NotEmpty().WithMessage("用户名不能为空")
    .MinimumLength(3).WithMessage("用户名至少3个字符")
    .MaximumLength(20).WithMessage("用户名最多20个字符");

    RuleFor(x => x.Email)
    .NotEmpty().WithMessage("邮箱不能为空")
    .EmailAddress().WithMessage("邮箱格式不正确");
}
}

// 注册验证器
builder.Services.AddValidatorsFromAssemblyContaining<CreateUserCommandValidator>();

2. 使用特性标记处理器


// 自定义特性
[AttributeUsage(AttributeTargets.Class)]
public class TransactionalAttribute:Attribute
{
}

// 创建事务行为
public class TransactionalBehavior<TRequest, TResponse>:IPipelineBehavior<TRequest, TResponse>
where TRequest:IRequest<TResponse>
{
private readonly ApplicationDbContext _context;

public TransactionalBehavior(ApplicationDbContext context)
{
        _context = context;
}

public async Task<TResponse>Handle(TRequest request,RequestHandlerDelegate<TResponse> next,CancellationToken cancellationToken)
{
// 检查是否标记了Transactional特性
if(request.GetType().GetCustomAttribute<TransactionalAttribute>()==null)
return await next();

await using var transaction =await _context.Database.BeginTransactionAsync(cancellationToken);

try
{
var response =await next();
await transaction.CommitAsync(cancellationToken);
return response;
}
catch
{
await transaction.RollbackAsync(cancellationToken);
throw;
}
}
}

3. 使用 MediatR 与 ASP.NET Core 集成

在控制器中使用 MediatR：


[ApiController]
[Route("api/users")]
public class UsersController:ControllerBase
{
privatereadonlyIMediator _mediator;

publicUsersController(IMediator mediator)
{
        _mediator = mediator;
}

[HttpPost]
publicasyncTask<IActionResult>CreateUser([FromBody]CreateUserCommand command)
{
var userId =await _mediator.Send(command);
returnCreatedAtAction(nameof(GetUser),new{ id = userId },null);
}

[HttpGet("{id}")]
publicasyncTask<ActionResult<UserDto>>GetUser(int id)
{
var query =newGetUserByIdQuery{ UserId = id };
var user =await _mediator.Send(query);
returnOk(user);
}
}