c# - Where 子句使用表达式 > 和

Question

这是我的代码:

public class Person
{
    public int Id {get; set;}
    public string Name {get; set;}
    public Place place {get; set;}
}

public class Customer : Person
{
    public int Id {get; set;}
    public string CustomerProperty {get; set;}
}

public class Employee : Person
{
    public int Id {get; set;}
    public string EmployeeProperty {get; set;}
}

所以我创建了一个方法来使用 linq Where 子句按房地产或城市或地点进行过滤。

        public static Expression<Func<Customer, bool>> FilterByPlace(string stateId, string cityId, string placeId)
    {
        if (placeId != null)
        {
            return u => u.PlaceId == placeId;
        }
        else if (cityId != null)
        {
            return u => u.Place.Address.cod_city == cityId;
        }
        else if (stateId != null)
        {
            return u => u.Place.Address.City.cod_state == stateId;
        }

        return u => true;
    }

如果我想为员工执行此操作，我需要复制并更改此 <Func<Customer>>至 <Func<Employee>> .有没有办法使用 <T> in 而不是我的实体？

当我更改 <Func<Person>> 的代码时它返回错误，因为 Type 与 linq 表达式的预期类型不同。

var custom = db.Customers
                            .Include(item => item.Place)
                            .Include(item => item.Place.Address)
                            .Include(item => item.Place.Address.City)
                            .Include(item => item.Place.Address.City.State)
                            .Where(item => item.Status == (Status)tableStatus)
                            .Where(FilterByPlace(stateId, cityId, placeId))
                            .OrderByDescending(item => item.CustomerId);

谢谢!

Answer 1

尝试创建一个以 Person 作为通用约束的通用方法。

public static Expression<Func<T, bool>> FilterByPlace<T>(string stateId, string cityId, string placeId) 
   where T : Person
{
    if (placeId != null)
    {
        return u => u.PlaceId == placeId;
    }
    else if (cityId != null)
    {
        return u => u.Place.Address.cod_city == cityId;
    }
    else if (stateId != null)
    {
        return u => u.Place.Address.City.cod_state == stateId;
    }

    return u => true;
}