Exception when try to modify my model - asp.net-mvc

Question

when I try to add in my model some note appear Exception
Here is my CompetitionRepository where I implement method
public void AddCompetition(Competition competition)
{
if (competition.ID == 0)
_context.Competitions.Add(competition);
else
_context.Entry(competition).State = EntityState.Modified;
_context.SaveChanges();
}
Controller
[HttpPost]
public ActionResult AdminPCreate(string compName, int quantity, DateTime starTime)
{
if(ModelState.IsValid)
_dataManager.Competitions.AddCompetition(
new Competition
{
ID = 1,
Quantity = quantity,
StartTime = starTime,
});
return View("Competitions",GetCompetitions());
}
And cshtml Page, maybe I'm doing something wrong
#using (Html.BeginForm("AdminPCreate", "Home"))
{
#Html.TextBox("compName",null ,new {#placeholder = "Competition Name"})
#Html.TextBox("quantity", null, new {#placeholder = "Amount players"})
#Html.TextBox("starTime", null, new {#placeholder = "Enter beginnig of the match like dd\\mm\\yyyy"})
<input type="submit" value="Create" class="btn btn-success"/>
I also tried using a lot of solutions including here on that site, such as that
because when i try to use that (ObjectStateManager.GetObjectStateEntry(entity)`
because my field methods have type different from object
public void AddCompetition(Competition competition)

Answer 1

In the method AdminPCreate of your controller, you have new Competition { ID = 1, [..] }.
That ID valueof 1 makes your repository think it's an existing item, so Entity Framework tries to update the Competition record where ID = 1. That doesn't exist, so your database returns "0 rows affected" and the error is thrown.
I suspect that when you set the ID to 0 instead of 1 in your controller it'll work.