gistfile1.cs

public interface IMapper<TInput, TOutput>
{
	TOutput Map(TInput input);
}

public interface ICompositeMapper<TInput, TOutput>
{
	TOutput[] Map(TInput input); 
}

// Implementation

public class Mapper : IMapper<XModel, Model>
{

	public Model Map(XModel)
	{
	
	}
}

public class CompositeMapper : ICompositeMapper<XModel, Model>
{

	public Model[] Map(XModel)
	{
	
	}
}

// Tests

public class MapperTests
{
	[Test]
	public void should_map()
	{
		var mapper = new Mapper();
		var result = mapper.Map(input);
		// assert result
	}
}

public class CompositeMapperTests
{
	[Test]
	public void should_map()
	{
		var mapper = new CompositeMapper();
		var result = mapper.Map(input);
		// assert result
	}
}

// Use case

public class SomeRepository : IRepository
{
        public Model GetById (int id)
	{
		var xModel = // ask NHibernate for it..
		return new Mapper().Map(xModel);
	}
}