README.md

MetaMerge - documentation

Purpose

The purpose of MetaMerge is to allow developers to keep sets of attributes consistent across different classes.

Tutorials

Still to do, but the installation instructions are simple. Add a package reference to Fody and then to MetaMerge.Fody on the project you want attributes merged into.

Problem definition

The following class is an example of a simple Entity named Person which has Salutation, GivenName, and FamilyName properties. Each of these properties are decorated with various .net attributes; these are usually used by frameworks such as Entity Framework Core when generating a database.

Server entities

namespace MyApp.Entities
{
	public class Person
	{
		public Guid Id { get; set; }

		[Required, MinLength(2), MaxLength(8)]
		[Display(Name = "Salutation"]
		public string Salutation { get; set; }

		[Required, MinLength(2), MaxLength(32)]
		[Display(Name = "Given name")]
		public string GivenName { get; set; }

		[Required, MinLength(1), MaxLength(64)]
		[Display(Name = "Family name")]
		public string FamilyName { get; set; }
	}
}

And the following class is an example of a data-transfer-object (DTO) that is transferred via a web API between the server and the client.

Shared contract classes

namespace MyApp.Contracts
{
	public class Person
	{
		public Guid Id { get; set; }

		[Required, MinLength(2), MaxLength(8)]
		[Display(Name = "Salutation"]
		public string Salutation { get; set; }

		[Required, MinLength(1), MaxLength(64)]
		[Display(Name = "Family name")]
		public string FamilyName { get; set; }

		[Required, MinLength(1), MaxLength(32)]
		[Display(Name = "Given name")]
		public string GivenName { get; set; }
	}
}

The problem with the code above is that the Contract class defines the MinLength of FamilyName as 1 character, whereas the Entity defines the MinLength as 2 characters.

With properly robust validation in both our client and server, we would expect the client to accept an input of a single character X for a person's FamilyName, the API endpoint on the server will also determine the DTO is valid, but then validation on Domain entities will throw an exception because X is not at least 2 characters in length.

Solution

Instead, we can define a pattern of .net attributes in a single place, and then apply them to multiple targets.

Shared definitions

namespace MyApp.Definitions
{
	public static class PersonSalutation
	{
		[Required, MinLength(2), MaxLength(8)]
		[Display(Name = "Salutation"]
		public static object Target { get; set; }
	}

	public static class PersonGivenName
	{
		[Required, MinLength(2), MaxLength(32)]
		[Display(Name = "Given name")]
		public static object Target { get; set; }
	}

	public static class PersonFamilyName
	{
		[Required, MinLength(1), MaxLength(8)]
		[Display(Name = "Salutation"]
		public static object Target { get; set; }
	}
}

Server entities

namespace MyApp.Entities
{
	public class Person
	{
		public Guid Id { get; set; }

		[Meta(typeof(Definitions.PersonSalutation)]
		public string Salutation { get; set; }

		[Meta(typeof(Definitions.PersonGivenName)]
		public string GivenName { get; set; }

		[Meta(typeof(Definitions.PersonFamilyName)]
		public string FamilyName { get; set; }
	}
}

Shared contract classes

namespace MyApp.Contracts
{
	public class Person
	{
		public Guid Id { get; set; }

		[Meta(typeof(Definitions.PersonSalutation)]
		public string Salutation { get; set; }

		[Meta(typeof(Definitions.PersonGivenName)]
		public string GivenName { get; set; }

		[Meta(typeof(Definitions.PersonFamilyName)]
		public string FamilyName { get; set; }
	}
}

Outcome

For each property MetaMerge finds that is decorated with a MetaAttribute it will replace it with the attributes defined on the Target property of the class specified.

Before

[Meta(typeof(Definitions.PersonSalutation)]
public string Salutation { get; set; }

After

[Required, MinLength(2), MaxLength(8)]
[Display(Name = "Salutation")]
public string Salutation { get; set; }