如何通过使用 C# 中的 Linq 比较另一个列表中的值来过滤列表？

Question

I have a list of Fruit which has name and price value.

public class Fruit {
    public string name {get; set;}
    public int price {get; set;}

    public Fruit(string n, int p)
    {
            name = n;
            price = p;
    }
    ...
}

List<Fruit> myBasket = new List<Fruit>();
myBasket.Add(new Fruit("apple", 300));
myBasket.Add(new Fruit("banana", 200));
myBasket.Add(new Fruit("kiwi", 400));

Also, there is a list of average market price of fruits.

List<Fruit> averagePrice = new List<Fruit>();
averagePrice.Add(new Fruit("apple", 500));
averagePrice.Add(new Fruit("banana", 100));
averagePrice.Add(new Fruit("kiwi", 600));

What I want to do is to filter myBasket to have only fruit that is lower price than averagePrice.

So, in the example, myBasket will have apple and kiwi.

I tried myBasket = myBasket.Where(v => v.price < averagePrice.FirstOrDefault(y => y.name.Equals(v.name)).price).ToList(); .

It works well, but has a possibility of null pointer exception when there is new fruit in myBasket but not in averagePrice .

How can I change it to prevent it from exception?

Thanks in advance.

Answer 1

This seems to be quite a simple way to do it:

myBasket =
(
    from f in myBasket
    join a in averagePrice on f.name equals a.name
    where f.price < a.price
    select f
).ToList();

That gives me:

---

Here's the equivalent method-syntax:

myBasket =
    myBasket
        .Join(averagePrice, f => f.name, a => a.name, (f, a) => new { f, a })
        .Where(x => x.f.price < x.a.price)
        .Select(x => x.f)
        .ToList();

Answer 2

Comprehension query syntax

Func<Fruit, bool> take = (fruit) =>
{
  var query = averagePrice.Where(f => f.name == fruit.name);
  return query.Count() == 0 ? true : fruit.price <= query.Min(f => f.price);
};

var result = from fruit in myBasket
             where take(fruit)
             orderby fruit.name
             select fruit;

foreach ( var item in result )
  Console.WriteLine($"{item.name}: {item.price}");

For each fruit in the basket we search it in the average list and take the minimum price available to compare it.

It may be possible to optimize this query with advanced functions like joins and projections or other...

Output

apple: 300
kiwi: 400

Output without average banana

apple: 300
banana: 200
kiwi: 400

Lambda query version

var result = myBasket.Where(fruit =>
{
  var query = averagePrice.Where(f => f.name == fruit.name);
  return query.Count() == 0 ? true : fruit.price <= query.Min(f => f.price);
});

foreach ( var item in result.OrderBy(f => f.name) )
  Console.WriteLine($"{item.name}: {item.price}");