C＃解析通用类型

Question

I am playing around with a test project. I am trying to implement a CQS pattern and I am getting close to wrapping up the initial tests. I have run into an issue with trying to resolve my IQueryValidtor<> and IQueryHandler<,> classes. The method resolves them alright, but when i try to access the interface methods implemented in the concrete class, i get

The best overloaded method match for 'MyProjectsNamespace.GetSiteValidator.Validate(MyProjectsNamespace.Queries.GetSite)' has some invalid arguments.

I am basing my code on this answer I found . Everything appears to be lining up during design time, but run time is a different story.

I am including all of the interfaces, implementations, and unit tests that I am working with on this issue. The first unit test is actually working. It is the last two that are actually trying to use the resolved classes. I am using Autofac for my dependency injection.

public interface IQuery<TResult> {
}

public class GetSite : IQuery<Site> {
    public Guid Id { get; set; }
}

public interface IValidator<T> {
    Task<List<ValidationResult>> Validate(T command);
}

public class GetSiteValidator : IValidator<GetSite> {
    public async Task<List<ValidationResult>> Validate(GetSite command) {
        List<ValidationResult> results = new List<ValidationResult>();

        if(command == null) {
            throw new ArgumentNullException(nameof(command));
        }

        if(command.Id == Guid.Empty) {
            results.Add(new ValidationResult() { FieldName = "Id", Message = "Is empty" });
        }

        return results;
    }
}

public interface IQueryHandler<in TQuery, TResult> where TQuery : IQuery<TResult> {
    Task<TResult> Handle(TQuery query);
}

public class GetSiteHandler : IQueryHandler<GetSite, Site> {
    public Task<Site> Handle(GetSite query) {
        throw new NotImplementedException();
    }
}

The code above is all of the interfaces and concrete classes used in this problem. The code below is the dispatcher class where i am having the issue.

public class QueryDispatcher : IQueryDispatcher {
    private readonly IComponentContext _context;

    public QueryDispatcher(IComponentContext context) {
        _context = context;
    }

    public async Task<TResult> Dispatch<TResult>(IQuery<TResult> query) {
        if (query == null) {
            throw new ArgumentNullException(nameof(query), "Query cannot be null");
        }

        // use dynamic datatype because the tresult is not known at compile time
        var validatorType = typeof(IValidator<>).MakeGenericType(query.GetType());
        dynamic validator = _context.Resolve(validatorType);

        var handlerType = typeof(IQueryHandler<,>).MakeGenericType(query.GetType(), typeof(TResult));
        dynamic handler = _context.Resolve(handlerType);

        List<ValidationResult> errors = await validator.Validate(query);

        if(errors.Count == 0) {
            return await handler.Handle(query);
        } else {
            // raise failed validation event
            throw new ApplicationException("Not implemented");
        }
    }
}

Answer 1

You are passing an IQuery<TResult> query object to the Validate(GetSite) method - the method requires a GetSite type.

Even though GetSite implements IQuery<TResult> , The compiler can't guarantee that IQuery<TResult> query is an object of type GetSite - it could also be some other type implementing IQuery<TResult> where a cast is not possible.