如何使用 Linq to Entity 获取最大 ID？

Question

I have a table User which has an identity column UserID , now what is the correct Linq to Entity line of code that would return me the max UserID ?

I've tried:

using (MyDBEntities db = new MyDBEntities())
{
    var User = db.Users.Last();
    // or
    var User = db.Users.Max();

    return user.UserID;
}

but Last and Max don't seem to be supported.

Any ideas?

Answer 1

这样做

db.Users.OrderByDescending(u => u.UserId).FirstOrDefault();

Answer 2

try this

int intIdt = db.Users.Max(u => u.UserId);

Update:

If no record then generate exception using above code try this

int? intIdt = db.Users.Max(u => (int?)u.UserId);

Answer 3

var max = db.Users.DefaultIfEmpty().Max(r => r == null ? 0 : r.ModelID);

当 db 中没有记录时，它将毫无例外地返回 0。

Answer 4

NisaPrieto

Users user = bd.Users.Where(u=> u.UserAge > 21).Max(u => u.UserID); 

will not work because MAX returns the same type of variable that the field is so in this case is an INT not an User object.

Answer 5

如果您使用async和await功能，则如下所示：

User currentUser = await db.Users.OrderByDescending(u => u.UserId).FirstOrDefaultAsync();

Answer 6

Note that none of these answers will work if the key is a varchar since it is tempting to use MAX in a varchar column that is filled with "ints".

In a database if a column eg "Id" is in the database 1,2,3, 110, 112, 113, 4, 5, 6 Then all of the answers above will return 6.

So in your local database everything will work fine since while developing you will never get above 100 test records, then, at some moment during production you get a weird support ticket. Then after an hour you discover exactly this line "max" which seems to return the wrong key for some reason....

(and note that it says nowhere above that the key is INT...) (and if happens to end up in a generic library...)

So use:

Users.OrderByDescending(x=>x.Id.Length).ThenByDescending(a => a.Id).FirstOrDefault();

Answer 7

The best way to get the id of the entity you added is like this:

public int InsertEntity(Entity factor)
{
    Db.Entities.Add(factor);
    Db.SaveChanges();
    var id = factor.id;
    return id;
}