Does DbContext.AddAsync throw an exception given a duplicate unique index? - entity-framework-core

Question

I wrote the following code to add a Person to a database, and I am questioning the need to check for an existing email (the unique key) before adding the person.
Will AddAsync throw an exception for me if I try to add a Person with the same email as an existing Person?
I've included a MVCE including my Service, Repository, and DbContext, but really this is just about the behavior of one call and whether or not it throws an Exception. If so, what exception is it?
namespace Project
{
public sealed class PersonService
{
private readonly PersonRepository personRepository;
public PersonService(PersonRepository personRepository)
{
this.personRepository = personRepository;
}
public async Task<Person> AddAsync(Person person)
{
var existingPerson = await this.personRepository.GetByEmailAsync(person.Email);
if (existingPerson != null)
{
throw new DuplicateEmailException(person.Email, $"The Email {person.Email} is already taken.");
}
await this.personRepository.AddAsync(person);
return person;
}
}
public sealed class PersonRepository
{
private readonly ProjectDbContext dbContext;
public PersonRepository(ProjectDbContext dbContext)
{
this.dbContext = dbContext;
}
public async Task<Person> GetByEmailAsync(string email)
{
return await this.dbContext.Person
.FirstOrDefaultAsync(p => p.Email == email);
}
public async Task AddAsync(Person person)
{
if (person == null)
{
return;
}
await this.dbContext.AddAsync(person);
await this.dbContext.SaveChangesAsync();
}
}
public sealed class ProjectDbContext : DbContext
{
public ProjectDbContext(DbContextOptions options)
: base(options)
{
}
public DbSet<Person> Person { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<Person>()
.Property(p => p.Email)
.IsRequired();
modelBuilder.Entity<Person>()
.HasIndex(p => new { p.Email })
.IsUnique();
}
}
}

Answer 1

I am questioning the need to check for an existing email (the unique key) before adding the person. Will AddAsync throw an exception for me if I try to add a Person with the same email as an existing Person?
dbContext.AddAsync(person) will not throw any error but when _dbContext.SaveChanges(); will be called following exception will be thrown:
SqlException: Cannot insert duplicate key row in object 'dbo.Persons' with unique index 'IX_Perosns_Email'. The duplicate key value is (foo#gmail.com).
The statement has been terminated.
So you should validate/check this in controller or model level instead of in your PersonService.AddAsync() service method so that you can show the proper validation error message to the end user.