这个系列，我和大家一起来学学C#中一些非常有用函数，对于有些人来说可能它们不起眼，因此常常忽略它们。它们不会嚷嚷到：“使用我吧！我会让你节省很多的时间，而且让你的代码变得更加的简洁！” -- 但是，这些话会从对它们熟悉的程序员的脑海中涌现出来。
public sealed class Product
{
public int Id { get; set; }
public string Category { get; set; }
public double Value { get; set; }
public override string ToString()
{
return string.Format("[{0}: {1} - {2}]", Id, Category, Value);
}
}
public static List<Product> GetList()
{
var products = new List<Product>
{
new Product {Id = 1, Category = "Electronics", Value = 15.0},
new Product {Id = 2, Category = "Groceries", Value = 40.0},
new Product {Id = 3, Category = "Garden", Value = 210.3},
new Product {Id = 4, Category = "Pets", Value = 2.1},
new Product {Id = 5, Category = "Electronics", Value = 19.95},
new Product {Id = 6, Category = "Pets", Value = 21.25},
new Product {Id = 7, Category = "Pets", Value = 5.50},
new Product {Id = 8, Category = "Garden", Value = 13.0},
new Product {Id = 9, Category = "Automotive", Value = 10.0},
new Product {Id = 10, Category = "Electronics", Value = 250.0},
};
return products;
}
foreach (var group in products.GroupBy(p => p.Category))
{
Console.WriteLine(group.Key);
foreach (var item in group)
{
Console.WriteLine(" " + item);
}
}
当我们使用 GroupBy（） 扩展方法时，使用了延迟执行。 这意味着，当你遍历集合的时候,下一个要出现的项目可能会或者可能不会被加载。 这是一个很大的性能改进，但它会引起有趣的副作用。
