Setting the identity of a Domain Entity

Entity.cs

	public class Entity
	{
		private int? id;

		public int Id
		{
			get
			{
				return id.Value;
			}
		}

		public Entity()
		{ }

		public static void SetId(Entity entity, int id)
		{
			entity.id = id;
		}
	}

EntityHelper.cs

	public static class EntityHelper
	{
		/// <summary>
		/// Sets the id of the entity using reflection.
		/// </summary>
		/// <typeparam name="TEntity">The type of the entity.</typeparam>
		/// <param name="entity">The entity with Id property.</param>
		/// <param name="id">The id value to be set to.</param>
		/// <returns>
		/// Entity with updated 'Id'
		/// </returns>
		public static TEntity WithId<TEntity>(this TEntity entity, int id)
			where TEntity : Entity
		{
			SetId(entity, id);
			return entity;
		}

		/// <summary>
		/// Sets the id of the entity using reflection.
		/// </summary>
		/// <typeparam name="TEntity">The type of the entity.</typeparam>
		/// <param name="entity">The entity with Id property.</param>
		/// <param name="id">The id value to be set to.</param>
		/// <exception cref="System.Exception">'id' private field cannot be found.</exception>
		private static void SetId<TEntity>(TEntity entity, int id)
			where TEntity : Entity
		{
			var idProperty = GetField(entity.GetType(), "id", BindingFlags.NonPublic | BindingFlags.Instance);

			if (idProperty == null)
			{
				throw new Exception("'id' private field cannot be found.");
			}

			idProperty.SetValue(entity, id);
		}

		/// <summary>
		/// Gets the field info.
		/// </summary>
		/// <param name="type">The type.</param>
		/// <param name="fieldName">Name of the field.</param>
		/// <param name="bindibgAttr">Specifies flags that control binding and the way in which the search for members and types is conducted by reflection.</param>
		/// <returns>
		/// Null or found field info.
		/// </returns>
		public static FieldInfo GetField(Type type, string fieldName, BindingFlags bindibgAttr)
		{
			return type != null
				? (type.GetField(fieldName, bindibgAttr) ?? GetField(type.BaseType, fieldName, bindibgAttr))
				: null;
		}
	}

SetIdentityTests.cs

	[TestClass]
	public class SetIdentityTests
	{
		[TestMethod]
		public void SetIdentity_ViaStaticMethod()
		{
			var target = new Entity();
			Entity.SetId(target, 100500);
			target.Id.Should().Be(100500);
		}

		[TestMethod]
		public void SetIdentity_ViaReflection()
		{
			var target = new Entity().WithId(100500);
			target.Id.Should().Be(100500);
		}
	}