WIP creational patterns

creational/builder/FunctionalBuilderSample.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace design_patterns.creational.functionalbuilder
+{
+    /// <summary>
+    /// Builder is a creational design pattern that 
+    /// lets you construct complex objects step by step. 
+    /// The pattern allows you to produce different types 
+    /// and representations of an object using the same construction code.
+    /// 
+    /// Functional builder - allow builder extension via extended methods
+    /// </summary>
+    public class FunctionalBuilderSample
+    {
+        public static async Task Run() {
+            Console.WriteLine("Creational - Functional Builder");
+
+            var product = new ProductBuilder()
+                .WithName("New Product")
+                .WithCategory("Best category")
+                .WithAge(30)
+                .WithPrice(45.22m)
+                .Build();
+            
+                Console.WriteLine("product: " +  product.ToString());
+        }
+    }
+
+    public abstract class FunctionalBuilderBase<T, SELF> 
+        where SELF : FunctionalBuilderBase<T, SELF>
+        where T : new()
+    {
+        private readonly List<Func<T, T>> actions = new List<Func<T, T>>();
+
+        public SELF Apply(Action<T> action) {
+            actions.Add(t => {
+                action(t);
+                return t;
+            });
+            return this as SELF;
+        }
+
+        public T Build() {
+            return actions.Aggregate(new T(), (t, f) => f(t));
+        }
+
+    }
+    public sealed class ProductBuilder : FunctionalBuilderBase<Product, ProductBuilder> {
+        public ProductBuilder WithName(string name) =>
+            Apply(p => p.ProductName = name);
+    }
+
+    /// <summary>
+    /// allow extending the builder
+    /// </summary>
+    public static class ProductBuilderExtensions {
+        public static ProductBuilder WithAge(this ProductBuilder builder, int age) =>
+            builder.Apply(p => p.Age = age);
+        public static ProductBuilder WithCategory(this ProductBuilder builder, string category) =>
+            builder.Apply(p => p.Category = category);
+        public static ProductBuilder WithPrice(this ProductBuilder builder, decimal price) =>
+            builder.Apply(p => p.Price = price);
+    }
+
+    public class Product {
+        public int Age { get; set; }
+        public decimal Price { get; set; }
+        public string ProductName { get; set; }
+        public string Category { get; set; }
+        public string Info { get; internal set; }
+
+        public override string ToString()
+        {
+            return $"{ProductName} : {Category} : {Age} : {Price}";
+        }
+    }
+}