如何在LINQ C＃中从数据库中获取数据？

Question

how can i get latest "id" from my database and put this in textbox note: i have to return latest id from my database.

look at this method :

public void refresh()
{
    var query = from qc in db.tbl_viewClients select qc.id  ;
    int a = Convert.ToInt32(query.SingleOrDefault().ToString());
    txt_id.Text = Convert.ToString(a);
}

Answer 1

You should handle when you have no record because Max will throw InvalidOperationException .

var query = from qc in db.tbl_viewClients select qc.id;
if (query.Any())
{
    var max = query.Max();
    txt_id.Text = max.ToString();
    // do rest of work
}
else
{
   //handle the case when you have no record
}

but this approach enumerates the query twice!
Here is a trick!

var query = from qc in db.tbl_viewClients select (int?)qc.id;
var max = query.Max();
if (max.HasValue)
{
    txt_id.Text = max.ToString();
    // do rest of work
}
else
{
    //handle the case when you have no record
}

Answer 2

Assuming that id is stored as int and latest means largest id , you can simply use Max() * :

int a = query.DefaultIfEmpty(0).Max();

Otherwise, information to determine latest , such as creation date , is missing from the question.

*) added DefaultIfEmpty(0) to return 0 in case query return no result.

Answer 3

Try this please.

var query = from t in table orderby t.Id descending
                    select t.Id;

object result =  query.FirstOrDefault();

Hope helps.

Answer 4

If you are trying to do what I think you are (inserting something into the database, and then querying it to find what you think is the just-inserted-ID), then stop! Doing this will not ensure that the ID you retrieve is the one you need, as your database can, at any time be changed by someone else (concurrency). But still, considering it's just a test, and it's just you who use it and you need to know the ID you just inserted, then the way to do this is:

Assuming you use LINQtoSQL or Entity Framework, when you add an object to the context and call SaveChanges(), right after this the object in question will have its ID (or primary key) field populated. Just check!