将可空引用类型与泛型类型一起使用时收到警告

Question

I have a generic type like the following, with a method named ExecuteAsync() which can return an object or null :

public interface IStoredProcedure<Result, Schema>
    where Result : IBaseEntity
    where Schema : IBaseSchema
{
    Task<Result> ExecuteAsync(Schema model);
}

public class StoredProcedure<Result, Schema> : IStoredProcedure<Result, Schema>
    where Result : IBaseEntity
    where Schema : IBaseSchema
{
    public async Task<Result> ExecuteAsync(Schema model){
        //I use QueryFirstOrDefaultAsync of Dapper here, which returns an object or null
        throw new NotImplementedException();
    }
}

I use this in my service as follows:

public interface IContentService
{
    Task<Content?> Get(API_Content_Get schema);
}

public class ContentService : IContentService
{
    private readonly IStoredProcedure<Content?, API_Content_Get> _api_Content_Get;

    public ContentService(IStoredProcedure<Content?, API_Content_Get> api_Content_Get)
    {
        _api_Content_Get = api_Content_Get;
    }

    public async Task<Content?> Get(API_Content_Get schema)
    {
        Content? result = await _api_Content_Get.ExecuteAsync(schema);
        return result;
    }
}

If I don't add the ? in ContentService to show that the content can be null , I get this warning:

I couldn't find a way to show that the content can be null . I can write it as follows, and it gets no warning, but it then assumes that the result value is not null :

private readonly IStoredProcedure<Content, API_Content_Get> _api_Content_Get;
public ContentService(IStoredProcedure<Content, API_Content_Get> api_Content_Get)
{
    _api_Content_Get = api_Content_Get;
}

public async Task<Content?> Get(API_Content_Get schema)
{
    Content? result = await _api_Content_Get.ExecuteAsync(schema);
    return result;
}

I know that it's just a warning and doesn't affect the process. But is there anything that I can do to fix it?

I think it's a bug in this new feature that should be fixed.

Answer 1

Looks like this is the syntax you're after:

public interface IStoreProcedure<Result, Schema>
where Result : IBaseEntity?
where Schema : IBaseSchema {
   Task<Result> ExecuteAsync(Schema model);
}

It seems that by default, in a nullable context, a type constraint implies non-nullability, so to get nullability you have to add ? to the type constraint.