.NET Core中如何实现方法返回多种可能类型？

问题场景 #

我正在使用.NET Core构建WebApi，尝试在数据访问层实现简单的注册校验逻辑：用户发送注册请求时，CheckCredentials服务会检查请求中需唯一的信息是否已存在于数据库中。若存在，应返回CCFeedback类型的结果；若不存在，则返回刚创建的User类型信息。

目前代码中直接抛出异常会导致服务器返回500内部错误，我希望用合适的响应替代异常，让CreateUser方法根据校验结果返回对应类型的数据。

原业务层代码（存在语法错误）：

// 当前报错：'the non generic type task cannot be used with type arguments'
public async Task<User || CCFeedback> CreateUser(User user)
{
    CCFeedback DAFeedback = await _UserRepository.CheckCredentials(user);
    switch (DAFeedback.property)
    {
        case "UO":
            throw new Exception("Username already taken!");
        case "PO":
            throw new Exception("You entered a registered phone!");
        case "UP":
            throw new Exception("Both Username and Phone are already registered!");
        case null:
            break;
        default:
            break;
    }
    return await _UserRepository.CreateUser(user);
}

数据访问层CheckCredentials方法：

public async Task<CCFeedback> CheckCredentials(User user)
{
    using (var rnTrainingDbContext = new RNTrainingDbcontext())
    {
        CCFeedback feedback;

        var entityUsernameCheck = await rnTrainingDbContext.Users
            .FirstOrDefaultAsync(entity => entity.Username == user.Username);
        var entityPhoneCheck = await rnTrainingDbContext.Users
            .FirstOrDefaultAsync(entity => entity.Phone == user.Phone);

        if((entityUsernameCheck != null) && (entityPhoneCheck != null))
        {
            feedback = new CCFeedback {
                property = "UP",  // UP -> Both username and phone already exists in DB
                isValid = false
            };
            return feedback;
        } else if ((entityUsernameCheck != null))
        {
            feedback = new CCFeedback {
                property = "UO",  // UO -> Only username already exists in DB
                isValid = false
            };
            return feedback;
        } else if ((entityPhoneCheck != null))
        {
            feedback = new CCFeedback {
                property = "PO",  // PO -> Only Phone already exists in DB
                isValid = false
            };
            return feedback;
        } else
        {
            feedback = new CCFeedback {
                property = null,
                isValid = true
            };
            return feedback;
        }
    }
}

解决方案 #

C#不支持直接让方法返回两种不同的类型，但可以通过以下几种方式实现类似效果：

1. 自定义返回包装类（推荐） #

创建一个专门的返回类，封装成功数据和错误信息，语义清晰且易于扩展：

首先定义包装类：

public class RegisterResponse
{
    // 注册成功时返回的用户信息
    public User CreatedUser { get; set; }
    // 注册失败时返回的校验反馈
    public CCFeedback ValidationError { get; set; }
    // 快速判断是否注册成功
    public bool IsSuccess => CreatedUser != null;
}

修改业务层CreateUser方法：

public async Task<RegisterResponse> CreateUser(User user)
{
    CCFeedback DAFeedback = await _UserRepository.CheckCredentials(user);
    
    if (DAFeedback.isValid)
    {
        var createdUser = await _UserRepository.CreateUser(user);
        return new RegisterResponse { CreatedUser = createdUser };
    }
    else
    {
        return new RegisterResponse { ValidationError = DAFeedback };
    }
}

最后在API控制器中处理返回结果，返回对应HTTP状态码：

[HttpPost("register")]
public async Task<IActionResult> Register(User user)
{
    var response = await _userService.CreateUser(user);
    
    if (response.IsSuccess)
    {
        return Ok(response.CreatedUser); // 200 OK + 用户信息
    }
    else
    {
        return BadRequest(response.ValidationError); // 400 BadRequest + 校验反馈
    }
}

2. 使用ValueTuple（快速原型场景） #

如果不想定义额外类，可以用C#的ValueTuple直接返回两个值，但语义化较差：

public async Task<(User CreatedUser, CCFeedback ErrorFeedback)> CreateUser(User user)
{
    CCFeedback DAFeedback = await _UserRepository.CheckCredentials(user);
    
    if (DAFeedback.isValid)
    {
        var createdUser = await _UserRepository.CreateUser(user);
        return (createdUser, null);
    }
    else
    {
        return (null, DAFeedback);
    }
}

控制器处理：

[HttpPost("register")]
public async Task<IActionResult> Register(User user)
{
    var (userResult, errorResult) = await _userService.CreateUser(user);
    
    if (userResult != null)
    {
        return Ok(userResult);
    }
    else
    {
        return BadRequest(errorResult);
    }
}

3. 优化校验反馈的类型安全性（可选） #

原代码中用字符串标识冲突类型容易出错，建议改用枚举提升类型安全：

// 定义冲突类型枚举
public enum CredentialConflictType
{
    None,
    UsernameOnly,
    PhoneOnly,
    BothUsernameAndPhone
}

// 修改CCFeedback类
public class CCFeedback
{
    public CredentialConflictType ConflictType { get; set; }
    public bool IsValid => ConflictType == CredentialConflictType.None;
}

数据访问层修改：

public async Task<CCFeedback> CheckCredentials(User user)
{
    using (var rnTrainingDbContext = new RNTrainingDbcontext())
    {
        var entityUsernameCheck = await rnTrainingDbContext.Users
            .FirstOrDefaultAsync(entity => entity.Username == user.Username);
        var entityPhoneCheck = await rnTrainingDbContext.Users
            .FirstOrDefaultAsync(entity => entity.Phone == user.Phone);

        var feedback = new CCFeedback();
        
        if (entityUsernameCheck != null && entityPhoneCheck != null)
        {
            feedback.ConflictType = CredentialConflictType.BothUsernameAndPhone;
        }
        else if (entityUsernameCheck != null)
        {
            feedback.ConflictType = CredentialConflictType.UsernameOnly;
        }
        else if (entityPhoneCheck != null)
        {
            feedback.ConflictType = CredentialConflictType.PhoneOnly;
        }
        else
        {
            feedback.ConflictType = CredentialConflictType.None;
        }
        
        return feedback;
    }
}

这样业务层判断时不用依赖字符串，避免拼写错误：

public async Task<RegisterResponse> CreateUser(User user)
{
    CCFeedback DAFeedback = await _UserRepository.CheckCredentials(user);
    
    if (DAFeedback.IsValid)
    {
        var createdUser = await _UserRepository.CreateUser(user);
        return new RegisterResponse { CreatedUser = createdUser };
    }
    else
    {
        return new RegisterResponse { ValidationError = DAFeedback };
    }
}

内容的提问来源于stack exchange，提问作者eaidy