The Curiosly Recursive Generic Pattern

crgp01 - product.cs

void Main()
{}

public class Manufacturer
{
	public Manufacturer(string id, string name)	{Id = id;Name = name;}
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp02 - simple product builder.cs

void Main()
{
}

public class ProductBuilder
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public ProductBuilder ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public ProductBuilder WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };
	public void Save()
	{
		if (manufacturer == null) throw new InvalidOperationException("Manufacturer should be set because foreign keys");
		DoSave(Build());
	}

	protected ProductBuilder Fluent(Action a) => this.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp03 - using the builder.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.Save();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

  //we want the compiler to tell us that the next line is invalid
	new ProductBuilder().Save();
  
}

public class ProductBuilder
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public ProductBuilder ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public ProductBuilder WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };
	public void Save()
	{
		if (manufacturer == null) throw new InvalidOperationException("Manufacturer should be set because foreign keys");
		DoSave(Build());
	}

	protected ProductBuilder Fluent(Action a) => this.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp04 -productSaver.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

  //This doesn't work anymore since only ProductSavers can save
	new ProductBuilder()
	.ManufacturedBy(dell)
	.Save();

 //This doesn't work anymore since only ProductSavers can save
	new ProductBuilder()
	.ManufacturedBy(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

 //This doesn't work anymore since only ProductSavers can save
	new ProductBuilder().Save();
}

public class ProductBuilder
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public ProductBuilder ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public ProductBuilder WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected ProductBuilder Fluent(Action a) => this.Fluent(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public class ProductSaver : ProductBuilder
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp05 - productSaver failed.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductSaver(dell)
	.Save();

	//This is invalid now, which is what we wanted
  new ProductSaver().Save();
  
  //but we broke this one too
  new ProductSaver(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();
}

public class ProductBuilder
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public ProductBuilder ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public ProductBuilder WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected ProductBuilder Fluent(Action a) => this.Fluent(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public class ProductSaver : ProductBuilder
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp06 - TSelf.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");
  
  //product builders are broken since it's now a generic class
	new ProductBuilder()
	.Build()
	.Dump();

  //product builders are broken since it's now a generic class
	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductSaver(dell)
	.Save();

	new ProductSaver(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

  //still not valid. good
	new ProductSaver().Save();
}

public class ProductBuilder<TSelf>
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public TSelf ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public TSelf WithSku(string aSku) => Fluent(() => sku = aSku);
	public TSelf Named(string aName) => Fluent(() => name = aName);
	public TSelf PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected TSelf Fluent(Action a) => this.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public class ProductSaver : ProductBuilder<ProductSaver>
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp07 - This.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductSaver(dell)
	.Save();

	new ProductSaver(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

	//still not valid. good
	//new ProductSaver().Save();
}

public abstract class ProductBuilder<TSelf>
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public TSelf ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public TSelf WithSku(string aSku) => Fluent(() => sku = aSku);
	public TSelf Named(string aName) => Fluent(() => name = aName);
	public TSelf PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected abstract TSelf This { get; }
	protected TSelf Fluent(Action a) => This.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public class ProductBuilder : ProductBuilder<ProductBuilder> { protected override ProductBuilder This => this; }

public class ProductSaver : ProductBuilder<ProductSaver>
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
	protected override ProductSaver This => this;
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp08 - Nonsense.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductSaver(dell)
	.Save();

	new ProductSaver(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

	new StupidProductBuilder()
		.Build();
    
    //broken because This returns a string so it breaks our chaining
   	new StupidProductBuilder()
 	  	.WithSku("0")
  		.Build();
		
  //still invaild. good 
	//new ProductSaver().Save();
}

public abstract class ProductBuilder<TSelf>
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public TSelf ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public TSelf WithSku(string aSku) => Fluent(() => sku = aSku);
	public TSelf Named(string aName) => Fluent(() => name = aName);
	public TSelf PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected abstract TSelf This { get; }
	protected TSelf Fluent(Action a) => This.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

//This is nonsense. It's valid, but string is not what we meant by TSelf
public class StupidProductBuilder : ProductBuilder<string> { protected override string This => "stupid"; }

public class ProductBuilder : ProductBuilder<ProductBuilder> { protected override ProductBuilder This => this; }

public class ProductSaver : ProductBuilder<ProductSaver>
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
	protected override ProductSaver This => this;
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp09 - f-bound.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductSaver(dell)
	.Save();

	new ProductSaver(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

  //still invalid. good
	//new ProductSaver().Save();
}

//Curiously Recursive Template Pattern CRTP    
//Curiously Recursive Generic Pattern CRGP
//                                          this where clause is an f-bound 
public abstract class ProductBuilder<TSelf> where TSelf : ProductBuilder<TSelf>
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	public TSelf ManufacturedBy(Manufacturer aManufacturer) => Fluent(() => manufacturer = aManufacturer);
	public TSelf WithSku(string aSku) => Fluent(() => sku = aSku);
	public TSelf Named(string aName) => Fluent(() => name = aName);
	public TSelf PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected abstract TSelf This { get; }
	protected TSelf Fluent(Action a) => This.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

//No longer valid. The f-bound prevents this nonsense
//public class StupidProductBuilder: ProductBuilder<string> { protected override string This => "stupid";}

//This is what we meant to happen
public class StupidProductBuilder: ProductBuilder<StupidProductBuilder> { protected override StupidProductBuilder This => this;}

//but I still can create stupid things like this. F-bound will only carry me until here.
public class StupidProductBuilder: ProductBuilder<ProductSaver> { protected override ProductSaver This => null;}

public class ProductBuilder : ProductBuilder<ProductBuilder> { protected override ProductBuilder This => this; }

public class ProductSaver : ProductBuilder<ProductSaver>
{
	public ProductSaver(Manufacturer aManufacturer) { manufacturer = aManufacturer; }
	public void Save() => DoSave(Build());
	protected override ProductSaver This => this;
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp10- Another way with Types.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.Save();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

	new ProductBuilder().Save();
}

public abstract class HasManufacturer { }

public class WithManufacturer : HasManufacturer { }

public class WithoutManufacturer : HasManufacturer { }

public class ProductBuilder<T> where T : HasManufacturer
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	protected ProductBuilder() { }
	private ProductBuilder(Manufacturer aManufacturer, string aSku, string aName, double aPrice)
	{
		manufacturer = aManufacturer;
		sku = aSku;
		name = aName;
		aPrice = price;
	}

	public ProductBuilder<WithManufacturer> ManufacturedBy(Manufacturer aManufacturer) => new ProductBuilder<WithManufacturer>(aManufacturer, sku, name, price);
	public ProductBuilder<T> WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder<T> Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder<T> PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };
	public void Save()
	{
		if (manufacturer == null) throw new InvalidOperationException("Manufacturer should be set because foreign keys");
		DoSave(Build());
	}

	protected ProductBuilder<T> Fluent(Action a) => this.Fluently(a);
	protected void DoSave(Product product) { product.Dump(); }
}

public class ProductBuilder : ProductBuilder<WithoutManufacturer> { }

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp11- Extension methods for the win.cs

void Main()
{
	var dell = new Manufacturer("m001", "DELL");

	new ProductBuilder()
	.Build()
	.Dump();

	new ProductBuilder()
	.WithSku("001")
	.Build()
	.Dump();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.Save();

	new ProductBuilder()
	.ManufacturedBy(dell)
	.WithSku("001")
	.Named("Xps laptop")
	.PricedAt(499.99)
	.Save();

  //now is invalid
	new ProductBuilder().Save();
}

public abstract class HasManufacturer { }

public class WithManufacturer : HasManufacturer { }

public class WithoutManufacturer : HasManufacturer { }

public class ProductBuilder<T> where T : HasManufacturer
{
	protected Manufacturer manufacturer;
	protected string sku;
	protected string name;
	protected double price;

	protected ProductBuilder() { }
	private ProductBuilder(Manufacturer aManufacturer, string aSku, string aName, double aPrice)
	{
		manufacturer = aManufacturer;
		sku = aSku;
		name = aName;
		aPrice = price;
	}

	public ProductBuilder<WithManufacturer> ManufacturedBy(Manufacturer aManufacturer) => new ProductBuilder<WithManufacturer>(aManufacturer, sku, name, price);
	public ProductBuilder<T> WithSku(string aSku) => Fluent(() => sku = aSku);
	public ProductBuilder<T> Named(string aName) => Fluent(() => name = aName);
	public ProductBuilder<T> PricedAt(double aPrice) => Fluent(() => price = aPrice);
	public Product Build() => new Product { Manufacturer = manufacturer, Sku = sku, Name = name, Price = price };

	protected ProductBuilder<T> Fluent(Action a) => this.Fluently(a);
}

public class ProductBuilder : ProductBuilder<WithoutManufacturer> { }

public static class ProductBuilderExtensions
{
	public static void Save(this ProductBuilder<WithManufacturer> builder) => DoSave(builder.Build());
	private static void DoSave(Product product) { product.Dump(); }
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t; }
}

public class Manufacturer
{
	public Manufacturer(string id, string name) { Id = id; Name = name; }
	public string Id { get; set; }
	public string Name { get; set; }
}

public class Product
{
	public string Sku { get; set; }
	public Manufacturer Manufacturer { get; set; }
	public string Name { get; set; }
	public double Price { get; set; }
}

crgp12 - Request example.cs

void Main()
{
	var r1= Request
			.ForAnalyticsByCountry
			.WithCountry("us");
	var r2= Request
			.ForOmnitureDataByCountryLanguage
			.WithCountry("us")
			.WithLanguage("en");
	var r3 =Request
			.ForClickDataByLwp
			.WithCountry("us")
			.WithLanguage("en")
			.WithSegment("gen")
			.WithCustomerSet("19");
	DummyDataRetriever.RetrieveDataWithFilters(r1);
	DummyDataRetriever.RetrieveDataWithFilters(r2);
	DummyDataRetriever.RetrieveDataWithFilters(r3);
}

public abstract class Request
{
	protected Dictionary<string, object> Filters = new Dictionary<string, object>();

	public static AnalyticsByCountry ForAnalyticsByCountry => new AnalyticsByCountry();
	public static OmnitureDataByCountryLanguage ForOmnitureDataByCountryLanguage => new OmnitureDataByCountryLanguage();
	public static ClickDataByLwp ForClickDataByLwp => new ClickDataByLwp();
	public static implicit operator Dictionary<string,object>(Request r) => r.Filters;
}

public abstract class Request<T>:Request where T:Request<T>
{
    protected T Fluent(Action a) => This.Fluently(a);
	protected abstract T This { get; }

	protected T Country    (string country    ) => Fluent(() => Filters["Country"    ] = country    );
	protected T Language   (string language   ) => Fluent(() => Filters["Language"   ] = language   );
	protected T Segment    (string segment    ) => Fluent(() => Filters["Segment"    ] = segment    );
	protected T CustomerSet(string customerSet) => Fluent(() => Filters["CustomerSet"] = customerSet);
}

public class AnalyticsByCountry : Request<AnalyticsByCountry>
{
	protected override AnalyticsByCountry This => this;
	public AnalyticsByCountry WithCountry(string country) => Country(country);
}

public class OmnitureDataByCountryLanguage : Request<OmnitureDataByCountryLanguage>
{
	protected override OmnitureDataByCountryLanguage This => this;
	public OmnitureDataByCountryLanguage WithCountry (string country ) => Country (country) ;
	public OmnitureDataByCountryLanguage WithLanguage(string language) => Language(language);
}

public class ClickDataByLwp : Request<ClickDataByLwp>
{
	protected override ClickDataByLwp This => this;
	public ClickDataByLwp WithCountry    (string country )    => Country    (country) ;
	public ClickDataByLwp WithLanguage   (string language)    => Language   (language);
	public ClickDataByLwp WithSegment    (string segment)     => Segment    (segment);
	public ClickDataByLwp WithCustomerSet(string customerSet) => CustomerSet(customerSet);
}

public class DummyDataRetriever
{
	public static object RetrieveDataWithFilters(Dictionary<string, object> filters) => filters.Dump();
}

public static class FluentExtensions
{
	public static T Fluently<T>(this T t, Action a) { a(); return t;}
}