Entity Framework auto generate GUID

Question

I am new to EF so here goes.I have a class which contains the following

public class EmailTemplate
{
    public Guid Id { get; set; }

    [MaxLength(2000)]
    public string Html { get; set; }
}

Here is my mapping class

class EmailMapper : EntityTypeConfiguration<EmailTemplate>
    {
        public EmailMapper()
        {
            ToTable("EmailTemplate");

            HasKey(c => c.Id);
            Property(c => c.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
            Property(c => c.Id).IsRequired();
        }
    }

I am trying to call DbContext.SaveChanges(), but I get the following error :

Exception Details: System.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'Id', table 'AutoSendConnection.dbo.EmailTemplates'; column does not allow nulls. INSERT fails.

What am i doing wrong? Why won't EF auto create a unique GUID?

Answer 1

Just decorate the Id field on your EmailTemplate class as below and SQL Server will automatically generate the value on insert.

[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
[Key]
public Guid Id { get; set; }

You can also remove your Mapper class as it's no longer needed.

Answer 2

If using .Net core then this should work for you ...

Use fluent API

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Node>().Property(x => x.ID).HasDefaultValueSql("NEWID()");
}

or

modelBuilder.Entity<Student>().Property(p => p.StudentID)
                .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

Here is a more comprehensive Cheat Sheet for entity framework