Question

I need some suggestions on how to build a fluent interface acting as a Builder, responsible for returning different concrete types depending on the methods called.

Imagine that I need to create one of the following types using my ProductBuilder (fluently): Product, ServiceProduct, PackagedProduct (both derived from Product).

I’m thinking of using a fluent syntax like this (other suggestions are more than welcome):

To create a Product:

var product = new ProductBuilder()
   .Create("Simple product")
   .WithPrice(12.5)

To create a ServiceProduct

var product = new ProductBuilder()
   .Create("Service product")
   .WithPrice(12.5)
   .AsServiceProduct()
       .ServiceProductSpecificMethods...()

And PackagedProduct with a call to AsPackagedProduct() instead of AsServiceProduct() etc. You get the idea.

I haven’t found a sample that shows best practices on this. Only samples where the final build returns the same type.

Any suggestions?

Answer 1

I see two options here.

If there are a finite number of products that are fixed, and not designed to be extended, then just create a Create method for each product:

var product = new ProductBuilder()
   .CreateSimple()
   .WithPrice(12.5);

var product = new ProductBuilder()
   .CreateService()
   .WithPrice(12.5)
   .ServiceProductSpecificMethods...();

If you don’t want (or can’t have) ProductBuilder to know all of the types of products, then I would use generics:

public class Product {}
public class SimpleProduct : Product {}
public class ServiceProduct : Product {}

var product = new ProductBuilder<SimpleProduct>()
   .WithPrice(12.5);

Here’s a starting place for the design to follow:

public class Product
{
    public decimal Price { get; set; }
}
public class SimpleProduct : Product { }
public class ServiceProduct : Product
{
    public string Service { get; set; }
}

public class ProductBuilder<T> where T : Product, new()
{
    private List<Action<T>> actions = new List<Action<T>>();

    public T Build()
    {
        T product = new T();
        foreach (var action in actions)
        {
            action(product);
        }

        return product;
    }
    public void Configure(Action<T> action)
    {
        actions.Add(action);
    }
}

public static class ProductExtensions
{
    public static ProductBuilder<T> WithPrice<T>(this ProductBuilder<T> builder, decimal price)
        where T : Product
    {
        builder.Configure(product => product.Price = price);
        return builder;
    }

    public static ProductBuilder<T> WithService<T>(this ProductBuilder<T> builder, string service)
            where T : ServiceProduct
    {
        builder.Configure(product => product.Service = service);
        return builder;
    }
}