生成过滤扩展方法

public static class MyExtensionMethods
    {
        public static IEnumerable<Product>FilterByCategory( this IEnumerable<Product> productEnum, string categoryParam) 
        {
            foreach(Product prod in productEnum) 
            {
                if(prod.Category == categoryParam) 
                {
                    yield return prod;
                }
            }
        }
    }

将一个集合 按照一定条件进行过滤 返回过滤之后的集合。

    ps：yield关键词把选择条件运用于源数据的条目，以产生一个结果子集。

调用方法：

Product[] productArray = {
                                            new Product {Name = "Kayak", Price = 275M},
                                            new Product {Name = "Lifejacket", Price = 48.95M},
                                            new Product {Name = "Soccer ball", Price = 19.50M},
                                            new Product {Name = "Corner flag", Price = 34.95M}
                                        };
 foreach(Product prod in productArray .FilterByCategory("Soccer")){
            Console.WriteLine("Name:{0}, Price {1:c}", prod.Name, prod.Price);
        }