C# SQL ID problem, how to check ID used

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Hi im new to C#
im trying to create a new costumer in my SQL database, when generating user ID to the SQL table, i have to check if ID is used, and if its used make error and set next number in the database as default for the new user, and show it to user before adding.

What I have tried:

// GET: Kundes/CreateOffentlig()
public ActionResult CreateOffentlig()
{
  ViewBag.KundeID = new SelectList(db.KundeOffentlig, "KundeID", "InstitutionsNavn");
  ViewBag.KundeID = new SelectList(db.KundePrivat, "KundeID", "PrivatMail");
  ViewBag.KundeID = new SelectList(db.KundeStorKunde, "KundeID", "FirmaNavn");
  Kunde kunde = new Kunde();
  kunde.KundeGr = 2;
  kunde.KundeID = db.Kunde.Max(p => p.KundeID) + 1;

  return View(kunde);
}

// POST: Kundes/Create
// To protect from overposting attacks, please enable the specific properties you want to bind to, for 
// more details see https://go.microsoft.com/fwlink/?LinkId=317598.
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult CreateOffentlig([Bind(Include = "KundeID,KundeGr,ForNavn,EfterNavn,Adresse1,Adresse2,PostNr,Mobil,InstitutionsNavn,CVRNummer,InstitutionsLeder,Mail")] Kunde kunde, KundeOffentlig kundeOffentlig)
{
  if (ModelState.IsValid) //Fejlhåndtering i forbindelse med id er brugt
  {
    try
    {
      kunde.KundeID = db.Kunde.Max(p => p.KundeID) + 1;
      kundeOffentlig.KundeID = kunde.KundeID;
      db.Kunde.Add(kunde);
      db.KundeOffentlig.Add(kundeOffentlig);
      db.SaveChanges();

      return RedirectToAction("Index");
    }
    catch (Exception)
    {
      ModelState.AddModelError("", "KundeID Already used ");

      return View(kunde);
    }
  }
}

Posted 25-Feb-19 3:19am

Thomas Kiær

Updated 25-Feb-19 21:32pm

Kornfeld Eliyahu Peter

v2

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Comments

F-ES Sitecore 25-Feb-19 9:21am

It's better to get the database to generate the new ID for you by using an identity column for the ID.

Kornfeld Eliyahu Peter 25-Feb-19 9:25am

https://docs.microsoft.com/en-us/sql/t-sql/statements/create-table-transact-sql-identity-property?view=sql-server-2017

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TheRealSteveJudge · Answer 1 · 2019-02-25T03:33:00

When creating a table make sure to have a primary key.
See explanation here SQL PRIMARY KEY Constraint[^]
If you do not want to recreate your table please follow the section

SQL PRIMARY KEY on ALTER TABLE

On table creation you should use Identity
e.g.

create table Example
(
   ID int IDENTITY(1,1) PRIMARY KEY,
   ...
)

as described in SQL AUTO INCREMENT a Field[^]

CHill60 · Answer 2 · 2019-02-25T03:59:00

Whereas Solution 1 will work to prevent duplicates, see the comment from S-ES Sitecore. Do not generate IDs by using "the next number" - what if two users try to update the table at the same time - you will get duplicate IDs.

You could use Identity - see Kornfeld Eliyahu Peter's comment IDENTITY (Property) (Transact-SQL) - SQL Server | Microsoft Docs[^]

Or you could use a sequence - The SEQUENCE, an updatable alternative for an IDENTITY column | Hans Michiels – Blog and more[^]

Thomas Kiær · Answer 3 · 2019-02-25T21:33:00

This worked for me :-)

if (ModelState.IsValid) //Fejlhåndtering i forbindelse med id er brugt
            {
                try
                {
                    
                    kundePrivat.KundeID = kunde.KundeID;
                    db.Kunde.Add(kunde);
                    db.KundePrivat.Add(kundePrivat);
                    db.SaveChanges();
                    return RedirectToAction("Index");
                }
                catch (Exception)
                {
                    int kundeID = db.Kunde.Max(p => p.KundeID) + 1;
                    ModelState.AddModelError("", "KundeID er allerede brugt. Nyt ID foreslået ;-)");
                    ModelState.SetModelValue("KundeID",
                        new ValueProviderResult(
                            kundeID,
                            kundeID.ToString(),
                            System.Globalization.CultureInfo.InvariantCulture
                        )
                    );
                    return View(kunde);

                }

C# SQL ID problem, how to check ID used

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