LINQ to Entities新手问题

Question

我不是一位经验丰富的 C# 程序员，目前我正在使用 EF 3.5 和 LINQ 做一些事情。

我有以下方法，并且我确信它可以用更好/更短的方式编写。

感谢您的帮助！

  public List<CustOrder> GetOrders(string supplierId, string locationId)
    {
        using (var ctx = new OrderEntities())
        {
            if (!string.IsNullOrEmpty(locationId))
            {
                var result = (from order in ctx.CustOrder
                              where order.SupplierId == supplierId
                                 && order.LocationId == locationId
                              select order).ToList();
                return result;
            }
            else
            {
                var result = (from order in ctx.CustOrder
                              where order.SupplierId == supplierId
                                 && order.LocationId != ""
                              select order).ToList();
                return result;              
            }
        }
    }

我的错误： 在第二个 linq 查询中，应删除以下行：

&& order.LocationId != ""

Answer 1

我会推荐这个版本，因为我发现它更具可读性。

public List<CustOrder> GetOrders(string supplierId, string locationId)
{
    using (var ctx = new OrderEntities())
    {
        var query = from order in ctx.CustOrder
                    where order.SupplierId == supplierId
                    select order;

        if (!string.IsNullOrEmpty(locationId))
        {
            query = query.Where(o => o.LocationId == locationId)
        }

        return query.ToList();
    }
}

Answer 2

你可以做

 public List<CustOrder> GetOrders(string supplierId, string locationId)
 {
        using (var ctx = new OrderEntities())
        {
             var result = (from order in ctx.CustOrder
                          where order.SupplierId == supplierId
                          && string.IsNullOrEmpty(locationId) ? true : order.LocationId == locationId
                                  select order).ToList();
             return result;
       }
 }

Answer 3

Bala 进行空值检查，Talljoe 进行空字符串检查。但这两者兼而有之：

 public List<CustOrder> GetOrders(string supplierId, string locationId)
 {
        using (var ctx = new OrderEntities())
        {
             var result = (from order in ctx.CustOrder
                                  where order.SupplierId == supplierId
                                     && ((String.IsNullOrEmpty(locationId) && order.LocationId == locationId) || 
                                         order.LocationId.Length > 0)
                                  select order).ToList();
             return result;
       }
 }

此外，检查字符串长度通常比针对空字符串进行相等性检查更好。

Answer 4

var result = (from order in ctx.CustOrder
                          where order.SupplierId == supplierId
                             && (String.IsNullOrEmpty(locationId)
                               || order.LocationId == locationId)
                          select order).ToList();
            return result;