如何使用LINQ选择与ID对应的名称

Question

I have a class points.cs which has members :

  public class Points
    {
        public int Id = 0;
        public string Name { get; set; }
    }


I have a list of these points like this `List<Points> list= new List<Points>();`

Now this list object contains data:

list:

 Id    Name
  1    abc
  2    def
  3    ghi
  4    jkl

What i have to do is to get the Name corresponding to the id number providing in my code using LINQ query from list object.

My try which os obviously wrong is:

 string nameFetchedId=list.Select(Name).Where(obj=>obj.Id=2) //result returned will be "def"

Please correct me, I am not good in LINQ ?

Answer 1

您的查询必须是：

string name = list.SingleOrDefault(obj => obj.Id == 2)?.Name; // ?. will prevent the code from throwing a null-reference exception and will return null when a point with Id = 2 does not exist

Answer 2

Greetings you may want to go like this using lambda expression:

string strName = list.where( x => x.id == 2).SingleOrDefault()?.Name; 

Goodluck

Answer 3

It should be

string nameFetchedId=list.SingleOrDefault(obj=>obj.Id==2)?.Name;

First you find your object, then select its Name :)

Answer 4

Selects first that meats the condition return null if not found

list.SingleOrDefault(obj=>obj.Id == 2); 
list.FirstOrDefault(obj=>obj.Id == 2);

in c# 6 use so you don't have to check if item is found

list.SingleOrDefault(obj=>obj.Id == 2)?.Name; 
list.FirstOrDefault(obj=>obj.Id == 2)?.Name;

this will return null or the Name value.

Selects first that meats the condition throws exception if not found

list.Single(obj=>obj.Id == 2);
list.First(obj=>obj.Id == 2);

with this it's safe to use list.Single(obj=>obj.Id == 2).Name; list.First(obj=>obj.Id == 2).Name; You will not get null unless the Name is null just an exception.

If you are using some sort of LINQ to data server (EF,NH,Mongo) some solutions will act a bit differently then when doing in memory query For more info on Single vs First check out LINQ Single vs First