Question

我的问题基于this SO帖子。这有助于我返回null，但是如何为特定字段返回null

实施例

select new FlowerMetaData
{
    FlowerId = flowers.FlowerId,
    UpdatedDate = flowers.UpdatedDate,
    FlowerDates = FlowerDates.OrderBy(d => d.CreatedDate).ToList(),
    ClosedDate = FlowerDates
        .Where(c => c.FlowerType == "Rainbow Rose" || c.FlowerType == "Black Rose" || c.FlowerType == "SunFlower" || c.FlowerType == "Lotus" || c.FlowerType == "Tulip" || c.FlowerType == "Rose")
        .Any()
           ? flowersDates.Where(c => c.FlowerType == "Rainbow Rose" || c.FlowerType == "Black Rose" || c.FlowerType == "SunFlower" || c.FlowerType == "Lotus" || c.FlowerType == "Tulip" || c.FlowerType == "Rose").OrderByDescending(x => x.CreatedDate).Select(x => x.CreatedDate).FirstOrDefault()
           : (DateTime?)null
}).FirstOrDefault();

我想仅在关闭日期返回null。

Answer 1

您只能使用FirsOrDefault：

...
ClosedDate = FlowerDates.Where(c =>new string[] {"Rainbow Rose", 
                                             "Black Rose",
                                             "SunFlower",
                                             "Lotus",
                                             "Tulip",
                                             "Rose"}.Contains( c.FlowerType))
                        .OrderByDescending(x => x.CreatedDate)
                        .Select(x =>(DateTime?) x.CreatedDate)
                        .FirstOrDefault(),

也是当你有这么多或条件好的时候使用Contains扩展方法。如果没有符合条件的元素，则结果为null。

Answer 2

您的代码基本上已经在运行，但是有很多令人费解的代码混淆了这个问题。

我认为你的问题归结为根据ClosedDate评估为真的有条件地设置Any()：

void Main()
{
    var FlowerDates = new List<Flower>();
    FlowerDates.Add(new Flower {FlowerId =1});//Comment this out and you will get NULL
    FlowerDates.Add(new Flower {FlowerId =2});

    //I simplified the condition to focus on choosing a null value
    //If Any() evaluates to true then get current time, else return null
    //Yes you are doing a projection here, but setting a variable is effectively the same thing and simpler to follow
    var ClosedDate = FlowerDates.Where(c => c.FlowerId == 1)
        .Any()
           ? DateTime.Now
           : (DateTime?)null;

    //ClosedDate is either NULL or DateTime.Now depending on Any()
}

public class Flower
{
    public int FlowerId { get; set; }
}

如何为特定字段重新设置null

2 个答案: