I just declared some code-first models for a new project that uses EntityFramework.
public class BlogEntry { public long Id { get; set; } public long
using System.Data.Entity;
It's in EF v4.1 and above, but you need a reference as it is an extension method.
Edit (thanks to @EastonJamesHarvey)
If using EF Core the import should be:
using Microsoft.EntityFrameworkCore;