How to validate models before SaveChanges in EntityFramework Core 2?

Yeah, looks like there is no out of the box validation. You can handle it by yourself by overriding SaveChanges:

public class MyContext : DbContext
{
    public virtual DbSet<Model> Models { get; set; }

    public MyContext()
    {
    }

    public override int SaveChanges()
    {
        var changedEntities = ChangeTracker
            .Entries()
            .Where(_ => _.State == EntityState.Added || 
                        _.State == EntityState.Modified);

        var errors = new List<ValidationResult>(); // all errors are here
        foreach (var e in changedEntities)
        {
            var vc = new ValidationContext(e.Entity, null, null);
            Validator.TryValidateObject(
                e.Entity, vc, errors, validateAllProperties: true);
        }

        return base.SaveChanges();
    }
}

public class Model
{
    [Key]
    public int Id { get; set; }

    [Required]
    [MaxLength(32)]
    public string Field { get; set; }

    [Range(15, 25)]
    public int RangeField { get; set; }
}

• Property validateAllProperties is important because without it you won't get range validations.
• Don't forget to override SaveChangesAsync.

This thing won't be really helpful if you set constraints with fluent syntax, unfortunately. Actually, as fluent syntax can possibly not only add new constraints, but also override existing, it's better to switch completely to attributes annotation, or find another way to make validation.