具有自定义 return 类型的通用 CQRS 查询处理程序

Question

我正在尝试使用 MediatR (v8) 库构建通用查询处理程序。让我们跳转到代码：首先，我有一个像这样的抽象查询 class：

public abstract class Query<TQueryResult> : IRequest<TQueryResult>
{
    public Guid Id { get; } = Guid.NewGuid();

    public DateTime Timestamp { get; }

    protected Query()
    {
        Timestamp = DateTime.Now;
    }
}

我想从相应的查询处理程序中 return 一个 Result 包装器对象，如下所示：

public class Result<T> 
{
    public T Payload { get;  }
    public string FailureReason { get;  }

    public bool IsSuccess => FailureReason == null;

    public Result(T payload)
    {
        Payload = payload;
    }

    public Result(string failureReason)
    {
        FailureReason = failureReason;
    }

    public static Result<T> Success(T payload)
        => new Result<T>(payload);

    public static Result<T> Failure(string reason)
        => new Result<T>(reason);

    public static implicit operator bool(Result<T> result) => result.IsSuccess;

}

最后但同样重要的是，让我们看看查询处理程序：

public abstract class AbstractQueryHandler<TQuery, TQueryResult, TResultValue> : IRequestHandler<TQuery, TQueryResult>
    where TQuery : Query<TQueryResult>
    where TQueryResult : class
{

    public Task<TQueryResult> Handle(TQuery request, CancellationToken cancellationToken)
    {
        try
        {
            return HandleQuery(request);
        }
        catch (Exception e)
        {
            return Task.FromResult(Result<TResultValue>.Failure(GetFailureMessage(e)) as TQueryResult);
        }
    }

    public abstract Task<TQueryResult> HandleQuery(TQuery request);

    private static string GetFailureMessage(Exception e)
    {
        return "There was an error while executing query: \r\n" + e.Message;
    }
}

老实说，由于查询处理程序中的三个类型参数，我对这个解决方案并不满意。让我们看看一些相应的测试来揭示我的担忧。首先是测试助手对象：

public class ExampleDto 
{
    public string Name { get; set; }
}

public class BasicQuery : Query<Result<ExampleDto>>
{

}

public class BasicQueryHandler : AbstractQueryHandler<BasicQuery, Result<ExampleDto>, ExampleDto>
{
    public override Task<Result<ExampleDto>> HandleQuery(BasicQuery request)
    {
        return Task.FromResult(Result<ExampleDto>.Success(new ExampleDto() { Name = "Result Name" }));
    }
}

然后是测试：

    [Fact]
    public async Task GivenBasicQuery_whenHandle_thenSuccessResultWithPayload()
    {
        var handler = new BasicQueryHandler();

        var result = await handler.Handle(new BasicQuery(), CancellationToken.None);

        Check.That(result.IsSuccess).IsTrue();
        Check.That(result.Payload.Name).IsEqualToValue("Result Name");
    }

正如您在 BasicQueryHandler 中看到的，在声明三种类型时存在某种重复，即 <BasicQuery, Result<ExampleDto>, ExampleDto>。这对我来说真的很可疑。我还尝试了很多其他的可能性，检查了互联网上的文章和 SO questions/answers 但无法提出更清洁的解决方案。我究竟做错了什么？是否可以将（查询处理程序的）类型参数的数量减少到 2 个？在此先感谢您的帮助！

Answer 1

基本上，我将 Result<> 从 class 声明中的类型参数移到了方法声明中。为了清楚起见，我还删除了接口（无论如何你都没有共享定义）。

public abstract class AbstractQueryHandler<TQuery, TQueryResult>
    where TQuery : Query<TQueryResult>
    where TQueryResult : class, new()
{

    public Task<Result<TQueryResult>> Handle(TQuery request, CancellationToken cancellationToken)
    {
        try
        {
            return HandleQuery(request);
        }
        catch (Exception e)
        {
            return Task.FromResult(Result<TQueryResult>.Failure(new TQueryResult(), GetFailureMessage(e)));
        }
    }

    public abstract Task<Result<TQueryResult>> HandleQuery(TQuery request);

    private static string GetFailureMessage(Exception e)
    {
        return "There was an error while executing query: \r\n" + e.Message;
    }
}

public class BasicQueryHandler : AbstractQueryHandler<BasicQuery, ExampleDto>
{
    public override Task<Result<ExampleDto>> HandleQuery(BasicQuery request)
    {
        return Task.FromResult(Result<ExampleDto>.Success(new ExampleDto() { Name = "Result Name" }));
    }
}

请注意，由于您需要在异常路径中新建一个 TQueryResult，因此必须将 new() 关键字添加到通用约束中。

具有自定义 return 类型的通用 CQRS 查询处理程序

Generic CQRS Query handler with custom return type

c#

.net

generics

cqrs

mediatr