c# - 正确使用'收益率'(Proper use of 'yield return')

Question

The yield keyword is one of those keywords in C# that continues to mystify me, and I've never been confident that I'm using it correctly.

(yield关键字是C＃中那些继续使我神秘化的关键字之一，而且我从未确信我正确使用它。)

Of the following two pieces of code, which is the preferred and why?

(以下两段代码中，哪个是首选，为什么？)

Version 1: Using yield return

(版本1：使用收益率返回)

public static IEnumerable<Product> GetAllProducts()
{
    using (AdventureWorksEntities db = new AdventureWorksEntities())
    {
        var products = from product in db.Product
                       select product;

        foreach (Product product in products)
        {
            yield return product;
        }
    }
}

Version 2: Return the list

(版本2：返回列表)

public static IEnumerable<Product> GetAllProducts()
{
    using (AdventureWorksEntities db = new AdventureWorksEntities())
    {
        var products = from product in db.Product
                       select product;

        return products.ToList<Product>();
    }
}

  ask by senfo translate from so

Answer 1

I tend to use yield-return when I calculate the next item in the list (or even the next group of items).

(当我计算列表中的下一个项目（甚至是下一组项目）时，我倾向于使用yield-return。)

Using your Version 2, you must have the complete list before returning.

(使用版本2，您必须在返回之前拥有完整列表。)

By using yield-return, you really only need to have the next item before returning.

(通过使用yield-return，您实际上只需要在返回之前拥有下一个项目。)

Among other things, this helps spread the computational cost of complex calculations over a larger time-frame.

(除此之外，这有助于在更大的时间范围内分散复杂计算的计算成本。)

For example, if the list is hooked up to a GUI and the user never goes to the last page, you never calculate the final items in the list.

(例如，如果列表连接到GUI并且用户永远不会转到最后一页，则永远不会计算列表中的最终项目。)

Another case where yield-return is preferable is if the IEnumerable represents an infinite set.

(另一种情况，其中yield-return是优选的，如果IEnumerable表示无限集。)

Consider the list of Prime Numbers, or an infinite list of random numbers.

(考虑素数列表，或无限的随机数列表。)

You can never return the full IEnumerable at once, so you use yield-return to return the list incrementally.

(您永远不能一次返回完整的IEnumerable，因此您使用yield-return以递增方式返回列表。)

In your particular example, you have the full list of products, so I'd use Version 2.

(在您的特定示例中，您有完整的产品列表，因此我将使用版本2。)