Why should you care?

ShopNest cannot save to the database negative prices or empty customer emails — validation returns friendly errors before SQL throws generic constraint violations.

See it live — copy this example

Paste into a .NET project with EF Core packages, then run with LocalDB/SQL Server (dotnet ef / dotnet run).

public class Product : IValidatableObject
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public decimal Price { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext ctx)
    {
        if (Price <= 0)
            yield return new ValidationResult("Price must be positive.", new[] { nameof(Price) });
    }
}

// Before save:
var errors = _context.ChangeTracker.Entries<Product>()
    .SelectMany(e => e.Properties.Select(p => e))
    .ToList();
_context.SaveChanges(); // throws DbUpdateException if annotations fail

What happened?

• IValidatableObject runs custom rules.
• EF can validate annotations on SaveChanges if configured.
• Invalid entities throw before SQL executes.

Practice next

1. Add [Required] and Range to Product and Customer.
2. build IValidatableObject for cross-field rules.
3. Catch DbUpdateException and map to ProblemDetails in API.
4. Add ISaveChangesInterceptor to validate all Added Product entries centrally.
5. Test invalid Price with TryValidateObject in a unit test without database.

Remember

Validate before SaveChanges when possible. Annotations and IValidatableObject integrate with EF. Combine with SQL constraints for integrity.

ShopNest seller upload

Bulk CSV import validates each Product row before AddRangeAsync.

Outcome: Bad rows rejected with row numbers instead of cryptic SQL errors.