c# - 使用 Entity Framework 核心构建动态查询 - 构建查询 "by steps"

Question

我正在尝试创建一个类来执行动态过滤器，这取决于我将发送给它的“过滤器”方法的参数，但我在构建查询时遇到了一些困难。

用下面的代码更容易解​​释，我只是尝试根据输入的内容应用一些过滤器:

public class PeopleManager : Controller
    {
        private readonly MyPeopleContext _context;
        public PeopleManager(PeopleContext context)
        {
            _context = context;
        }

        public IEnumerable<Person> filter (int? id, string name, int? age)
        {
            var peoplequery = _context.People;
            if(id!=null)
            {
               peoplequery = peoplequery .Where(p => p.Id.Equals(id));
            }

            if(name!="")
            {
               peoplequery = peoplequery .Where(p => p.Name.Contains(name));
            }

            if(age!=null)
            {
               peoplequery  = peoplequery .Where(p => p.Age.Equals(age));
            }

            return peoplequery;
        }
    }

显然，我的代码无法正常工作，根据我发现的这个问题:

Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Data.Entity.DbSet'

我已经设法按如下方式编辑我的代码:

public IEnumerable<Person> filter (int? id, string name, int? age)
            {
                var people = _context.People;

                var peoplequery = from p in people select people;

                if(id!=null)
                {
                   peoplequery = peoplequery .Where(p => p.Id.Equals(id));
                }

                if(name!="")
                {
                   peoplequery = peoplequery .Where(p => p.Name.Contains(name));
                }

                if(age!=null)
                {
                   peoplequery  = peoplequery .Where(p => p.Age.Equals(age));
                }

                return peoplequery;
            }

但是这样做，我无法在我的 lambda 表达式中访问人们的属性，例如 id、name 等。

我怎样才能不出错地构建这个条件查询？

Answer 1

我猜你想拥有这样的东西:

public class PeopleController : Controller
{
    private readonly MyPeopleContext _context;

    public PeopleManager(PeopleContext context)
    {
        _context = context;
    }

    public IEnumerable<Person> Filter (int? id, string name, int? age)
    {
        var peoplequery = _context.People.AsQueryable();
        if(id.HasValue)
        {
           peoplequery = peoplequery.Where(p => p.Id == id.Value);
        }
        if(!string.IsNullOrEmpty(name))
        {
           peoplequery = peoplequery.Where(p => p.Name.Contains(name));
        }
        if(age.HasValue)
        {
           peoplequery = peoplequery.Where(p => p.Age == age.Value);
        }

        return peoplequery.ToArray();
    }
}