Question

我正在使用Entity Framework 5，并且在创建要在方法中使用的表达式时遇到问题。

我认为问题在于通常我会在lambda表达式中调用表达式，例如dbContext.Counties.Select(GetLargeCities())，但在我正在使用的代码中，我将Counties实体投影到名为CountyWithCities的视图模型中。在我通常称之为表达式的地方，我有一个单例c，并且不知道如何在那里调用表达式。

我想使用表达式实现此目的的原因是因为我希望GetCountiesWithCities方法命中数据库一次，实体框架为结果中的所有对象构建一个复杂的图形。

由于某种原因，下面的代码产生错误`当前上下文中不存在名称'GetLargeCities'。“

    public class CountyWithCities // this is a view model
    {
        public int CountyID { get; set; }
        public string CountyName { get; set; }
        public IEnumerable<City> Cities { get; set; }
    }

    public class City // this is an entity
    {
        public int CityID { get; set; }
        public string CityName { get; set; }
        public int Population { get; set; }
    }

    public IEnumerable<CountyWithCities> GetCountiesWithCities(int StateID)
    {
        return dbContext.States
            .Where(s => s.StateID = StateID)
            .Select(s => s.Counties)
            .Select(c => new CountyWithCities
            {
                CountyID = c.CountyID,
                CountyName = c.CountyName,
                Cities = GetLargeCities(c) // How do I call the expression here?
            });
    }

    public Expression<Func<County, IEnumerable<City>>> GetLargeCities = county =>
        county.Cities.Where(city => city.Population > 50000);

谢谢！

Answer 1

我通常使用扩展方法执行此操作。

public static IQueriable<City> LargeCities(this IQueriable<County> counties){
    return counties
        .SelectMany(county=>county.Cities.Where(c=>c.Population > 50000));
}

用法：

dbContext.Counties.LargeCities()


   public IEnumerable<CountyWithCities> GetCountiesWithCities(int StateID)
    {
        return dbContext.States
            .Where(s => s.StateID = StateID)
            .Select(s => s.Counties)
            .LargeCities()
            .GroupBy(c=>c.County)
            .Select(c => new CountyWithCities
            {
                CountyID = g.Key.CountyID,
                CountyName = g.Key.CountyName,
                Cities = g.AsQueriable() // i cant remember the exact syntax here but you need the list from the group
            });
    }

C＃，调用Expression <func <t-in，t-out =“”>＆gt;的正确语法是什么？从另一个lambda表达式？</func <t-in，>

1 个答案: