How to implement Guid.Comb on SQL Server with Entity Framework Code First

Read this if you don’t know the “why” behind Guid.Comb. In a nutshell. We want to uniquely identify records and don’t want PK collisions across database instances. Using Guid as a PK used to be taboo. Now (circa 2002) it’s acceptable with Guid.Comb. It generates Guids in a semi-sequential order to limit page splits. This post is a combination of this and this.

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What I’m using:

EF 6 beta3 – Though you could probably use EF5 without issue

VS2012 – Express should work
SQL Express
Nuget

1. Create a new Console App called GuidComb

2. Open Package Manager Console and run the following:

PM> Install-Package EntityFramework -Pre

3. Add this code to your app:

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;

namespace GuidComb
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var db = new ItemContext())
            {
                for (var i = 0; i < 10; i++)
                    db.Items.Add(new Item());

                db.SaveChanges();
            }

            using (var db = new ItemContext())
            {
                foreach (var item in db.Items)
                    Console.WriteLine(item.Id);
            }

            Console.ReadKey();
        }
    }

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

    public class ItemContext : DbContext
    {
        public DbSet<Item> Items { get; set; }
    }
}

```text

4\. Run the following in Package Manager Console

> PM> Enable-Migrations

5\. Run the following in Package Manager Console

> PM> Add-Migration Comb

6\. Open the /Migrations/\[Timestamp\]\_Comb.cs that was created by the Add-Migration command and add defaultValueSql: “newsequentialid()” to the Item table Column builder.

```csharp

namespace GuidComb.Migrations
{
    using System;
    using System.Data.Entity.Migrations;

    public partial class Comb : DbMigration
    {
        public override void Up()
        {
            CreateTable(
                "dbo.Items",
                c => new
                    {
                        Id = c.Guid(nullable: false, identity: true, defaultValueSql: "newsequentialid()"),
                    })
                .PrimaryKey(t => t.Id);

        }

        public override void Down()
        {
            DropTable("dbo.Items");
        }
    }
}

7. Hit F5 and you should see this:

Screenshot

As you can see the Guid are being generated sequentially.

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