如何在LINQ中连接字符串，同时正确处理NULL值

Question

我想使用一种优雅的方法来使用LINQ将几个列连接在一起，但是当任何列为NULL时使用+操作符或concat()将导致连接后的值为NULL。

是否有类似于concat()处理NULL不同的东西，或者我在考虑这个不正确的方式?

任何帮助都是感激的!

下面是我使用的代码:

List<CustomObject> objects = (
    from obj in ObjectTable
    where obj.Id == Id
    select new CustomObject()
    {
        EnteredBy = obj.EnteredBy, 
        EntryDate = obj.EntryDate, 
        WorknoteText = 
            obj.VchWorkNote1 +
            obj.VchWorkNote2 + 
            obj.VchWorkNote3 +
            obj.VchWorkNote4 +
            obj.VchWorkNote5 +
            obj.VchWorkNote6 +
            obj.VchWorkNote7 +
            obj.VchWorkNote8 +
            obj.VchWorkNote9 +
            obj.VchWorkNote10 +
            obj.VchWorkNote11 +
            obj.VchWorkNote12 +
            obj.VchWorkNote13 +
            obj.VchWorkNote14 +
            obj.VchWorkNote15 +
            obj.VchWorkNote16 +
            obj.VchWorkNote17 +
            obj.VchWorkNote18 +
            obj.VchWorkNote19 +
            obj.VchWorkNote20
    }).ToList();

Answer 1

一种选择是使用空合并操作符:

List<CustomObject> objects = (from o in ObjectTable
                              where o.Id == Id
                              select new CustomObject(){
                              EnteredBy = o.EnteredBy, 
                              EntryDate = o.EntryDate, 
                              WorknoteText = 
                              (o.VchWorkNote1 ?? "") +
                              (o.VchWorkNote2 ?? "") + 
                              (o.VchWorkNote3 ?? "") +
                              (o.VchWorkNote4 ?? "") +
                              ...
                              (o.VchWorkNote20 ?? "")
                              }).ToList();

希望生成的SQL将使用适当的翻译

Answer 2

你可以使用??操作符:

...
(object.VchWorkNote5 ?? "") +
(object.VchWorkNote6 ?? "") +
(object.VchWorkNote7 ?? "") +
...

Answer 3

如何(object.VchWorkNote1 ?? "") +…

Answer 4

是否可以向数据库添加新列?比如"Keywords"或"FullText"

定义它有一个计算，这个计算基本上是"ISNULL(<Field1>, '') + ISNULL(<Field2>, '')"等

确保将其标记为持久化，这样就不必每次都计算。

然后你只需要从你的表中拉下一个字段

Answer 5

可能有一个更干净的方法，但首先想到的是一个快速的空到零长度字符串转换:

List<CustomObject> objects = (from object in ObjectTable
                         where object.Id == Id
                     select new CustomObject(){
                         EnteredBy = object.EnteredBy, 
                         EntryDate = object.EntryDate, 
                         WorknoteText = 
                         object.VchWorkNote1 ?? "" +
                         object.VchWorkNote2 ?? "" + 
                         object.VchWorkNote3 ?? "" +
                         object.VchWorkNote4 ?? "" +
                         object.VchWorkNote5 ?? "" +
                         object.VchWorkNote6 ?? "" +
                         object.VchWorkNote7 ?? "" +
                         object.VchWorkNote8 ?? "" +
                         object.VchWorkNote9 ?? "" +
                         object.VchWorkNote10 ?? "" +
                         object.VchWorkNote11 ?? "" +
                         object.VchWorkNote12 ?? "" +
                         object.VchWorkNote13 ?? "" +
                         object.VchWorkNote14 ?? "" +
                         object.VchWorkNote15 ?? "" +
                         object.VchWorkNote16 ?? "" +
                         object.VchWorkNote17 ?? "" +
                         object.VchWorkNote18 ?? "" +
                         object.VchWorkNote19 ?? "" +
                         object.VchWorkNote20 ?? ""
                     }).ToList();