constructor-ordinal-attribute.md

Constructor ordinal attribute

Applying this attribute disables automatic constructor selection. Only constructors marked with this attribute are considered, ordered by ordinal (ascending).

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .Arg<string>("connectionString")
    .Bind().To<Configuration>()
    .Bind().To<SqlDatabaseClient>()

    // Composition root
    .Root<IDatabaseClient>("Client");

var composition = new Composition(connectionString: "Server=.;Database=MyDb;");
var client = composition.Client;

// The client was created using the connection string constructor (Ordinal 0)
// even though the configuration constructor (Ordinal 1) was also possible.
client.ConnectionString.ShouldBe("Server=.;Database=MyDb;");

interface IConfiguration;

class Configuration : IConfiguration;

interface IDatabaseClient
{
    string ConnectionString { get; }
}

class SqlDatabaseClient : IDatabaseClient
{
    // The integer value in the argument specifies
    // the ordinal of injection.
    // The DI will try to use this constructor first (Ordinal 0).
    [Ordinal(0)]
    internal SqlDatabaseClient(string connectionString) =>
        ConnectionString = connectionString;

    // If the first constructor cannot be used (e.g. connectionString is missing),
    // the DI will try to use this one (Ordinal 1).
    [Ordinal(1)]
    public SqlDatabaseClient(IConfiguration configuration) =>
        ConnectionString = "Server=.;Database=DefaultDb;";

    public SqlDatabaseClient() =>
        ConnectionString = "InMemory";

    public string ConnectionString { get; }
}

dotnet --list-sdk

dotnet new console -n Sample

dotnet add package Pure.DI
dotnet add package Shouldly

dotnet run

The Ordinal attribute is part of the API, but you can define your own in any assembly or namespace.

partial class Composition
{
  private readonly string _argConnectionString;

  [OrdinalAttribute(128)]
  public Composition(string connectionString)
  {
    _argConnectionString = connectionString ?? throw new ArgumentNullException(nameof(connectionString));
  }

  public IDatabaseClient Client
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new SqlDatabaseClient(_argConnectionString);
    }
  }
}

Class diagram:

---
 config:
  class:
   hideEmptyMembersBox: true
---
classDiagram
	SqlDatabaseClient --|> IDatabaseClient
	Composition ..> SqlDatabaseClient : IDatabaseClient Client
	SqlDatabaseClient o-- String : Argument "connectionString"
	namespace Pure.DI.UsageTests.Attributes.ConstructorOrdinalAttributeScenario {
		class Composition {
		<<partial>>
		+IDatabaseClient Client
		}
		class IDatabaseClient {
			<<interface>>
		}
		class SqlDatabaseClient {
				<<class>>
			~SqlDatabaseClient(String connectionString)
		}
	}
	namespace System {
		class String {
				<<class>>
		}
	}

See also:

• Member ordinal attribute
• Dependency attribute