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Building Objects with Lambdas

Instead of allowing Lamar to build objects directly, you can give a Lamar Container a Lambda function that can be called to create an object at resolution time.

Using NHibernate's ISession as an example of an object that typically has to be built by using an ISessionFactory object:

public interface ISession { }

public interface ISessionFactory
    ISession Build();

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If we want to allow Lamar to control the ISession lifecycle and creation, we have to register a Lambda function as the means of creating ISession as shown in this example below:

public class SessionFactoryRegistry : Registry
    // Let's not worry about how ISessionFactory is built
    // in this example
    public SessionFactoryRegistry(ISessionFactory factory)

        // Build ISession with a lambda:
        For<ISession>().Use("Build ISession from ISessionFactory", c =>
            // To resolve ISession, I first pull out
            // ISessionFactory from the IContext and use that
            // to build a new ISession. 
            return c.GetInstance<ISessionFactory>().Build();

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Lambda registrations can be done with any of the following four signatures:

  1. (Expression<Func<IContext, T>> builder) -- a simple, one line Lambda to build T using IContext
  2. (Expression<Func<T>> func) -- a simple, one line Lambda to build T
  3. (string description, Func<IContext, T> builder) -- use IContext in your builder Lambda with a user-supplied description for diagnostics
  4. (string description, Func<T> builder) -- Supply a complex Func<T> with a user-supplied description for diagnostics

Be very wary of the difference between legal Expression's and more complicated Lambda's that will need to be Func's. It likely doesn't matter to you the user, but it unfortunately does to Lamar and .NET itself. If you need to use a more complex Func, you will have to supply a diagnostic description.