从对象列表中获取一个对象的属性

Question

I have a list of objects and then I want to find one object and display value of property with ID.

Names name1 = new Names(){ID=1, User="John"};
Names name2 = new Names(){ID=2, User="Mike"};
Names name3 = new Names(){ID=3, User="Ben"};

This is list these objects below:

List<Names> names;

Now I would like to return name of the User with ID equals 2. What inquiry I have to use when I have only list of objects?

Answer 1

You do it using one of First , FirstOrDefault , Single or SingleOrDefault dependng on your requirements,

I suspect the most appropriate is SingeOrDefault based on the fact an ID is usually unique, so you only ever expect one single item with a specified ID

var item = names.SingleOrDefault(x => x.ID == 2);
if(item != null){
    var name = item.Name;
}

The difference between Single and SingleOrDefault is that the former will thrw an error if not found, the latter will return the default value - null in the case of an object.

Answer 2

Try this

Names name1 = new Names() { ID = 1, User = "John" };
Names name2 = new Names() { ID = 2, User = "Mike" };
Names name3 = new Names() { ID = 3, User = "Ben" };

List<Names> names = new List<Names>();
names.Add(name1);
names.Add(name2);
names.Add(name3);

To find object with id=2

Names obj = names.Where(x => x.ID == 2).FirstOrDefault();
int ID = obj.ID;
string UserName = obj.User;

Answer 3

If you are not aware of the LINQ and then try this

Names name1 = new Names() { ID = 1, User = "John" };
Names name2 = new Names() { ID = 2, User = "Mike" };
Names name3 = new Names() { ID = 3, User = "Ben" };

List<Names> names = new List<Names>();
names.Add(name1);
names.Add(name2);
names.Add(name3);

foreach(var item in names)
{
  if(item.ID == 2)
  {
     string strName = item.Name;
     break;
  }
}

Answer 4

// this will return a list containing all names which are against user ID for example 1,
// or an empty list if there is no user with ID 1

var allnames = names.Where(u => u.ID == 1).ToList();