EF Core Exceptions Exceptions Handling

EntityFrameworkCore.Exceptions.SqlServer simplifies exceptions by handling all the database-specific details and throwing different exceptions.

• You have to configure the DbContext by calling UseExceptionProcessor in the OnConfiguring method and handle the exceptions.
• You can use UniqueConstraintException, CannotInsertNullException, MaxLengthExceededException, NumericOverflowException, ReferenceConstraintException as per your requirement.

So first, we need to update our context class, as shown below.

public class BookStore : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder
            .UseSqlServer(@"Data Source=(localdb)\ProjectsV13;Initial Catalog=BookStoreDb;")
            .UseExceptionProcessor();
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Author>().HasIndex(a => a.Name).IsUnique();
        builder.Entity<Author>().Property(a => a.Name).IsRequired().HasMaxLength(15);
    }

    public DbSet<Author> Authors { get; set; }
    public DbSet<Book> Books { get; set; }
}

You will now start getting a different exception for the different database errors. Let's consider the following example when a unique constraint fails, you will get the UniqueConstraintException exception as shown below.

using (BookStore context = new BookStore())
{
    try
    {
        var authors = new List<Author>
        {
            new Author { Name="Jones" },
            new Author { Name="Jones" }
        };

        authors.ForEach(a => context.Authors.Add(a));
        context.SaveChanges();
    }
    catch (UniqueConstraintException e)
    {
        Console.WriteLine(e.Message);
    }
}

When you execute the above code, you will see the following output.

Unique constraint violation

Let's consider another example where we have passed null for the Name property.

using (BookStore context = new BookStore())
{
    try
    {
        var author = new Author ();

        context.Authors.Add(author);
        context.SaveChanges();
    }
    catch (CannotInsertNullException e)
    {
        Console.WriteLine(e.Message);
    }
}

When you execute the above code, you will see the following output.

Cannot insert null