如何使用LINQ或Lambda代替嵌套和多个foreach语句

Question

How can I use LINQ or Lambda instead of nested and multiple foreach statements.

I want to use a better syntax than nested foreach statements to overwrite the initial list with items from the second list.

In the code below:

• I want to overwrite initialList with those in secondList that have the same Value. (Remove Red)

• Use the items in secondList where Value was the same (Yellow) New initialList list should include (Green and Yellow)

   static void Main(string[] args) { int useProd = 2; int useDomain = 0; var person1 = new Person() { prodId = 1, Value = "foo", domainId = 0, Name = "Red" }; var person2 = new Person() { prodId = 1, Value = "bar", domainId = 0, Name = "Green" }; var person3 = new Person() { prodId = 1, Value = "foo", domainId = 1, Name = "Yellow" }; var initialList = new List<Person>(); initialList.Add(person1); initialList.Add(person2); var secondList = new List<Person>(); secondList.Add(person3); List<Person> personsToRemove = new List<Person>(); List<Person> personsToUpdate = new List<Person>(); foreach (var pers1 in initialList) { foreach (var pers2 in secondList) { if (pers1.Value == pers2.Value) { personsToRemove.Add(pers1); personsToUpdate.Add(pers2); } } } foreach (var remPers in personsToRemove) { initialList.Remove(remPers); } foreach (var updPers in personsToUpdate) { initialList.Add(updPers); } foreach (var item in initialList) { Console.WriteLine(String.Format("Value: {0}, prodId: {1}, domainId: {2}, Name: {3}", item.Value, item.prodId, item.domainId, item.Name)); } Console.ReadKey(); } public class Person { public int prodId { get; set; } public string Value { get; set; } public int domainId { get; set; } public string Name { get; set; } } 

Answer 1

Your nested loops are most efficiently represented with a join . In addition, it would be helpful efficiency-wise to not have to do a linear-search on the entire list just to remove an element and then to add a new one in. There is an overload of Enumerable.Select() we can use to embed the item index in the result, so that the element can simply be replaced directly.

Putting it all together, it looks like this:

var join = from p1 in initialList.Select((p, i) => new { Person = p, Index = i })
           join p2 in secondList on p1.Person.Value equals p2.Value
           select new { Index = p1.Index, Replacement = p2 };

foreach (var item in join.ToList())
{
    initialList[item.Index] = item.Replacement;
}

The above code replaces the original code starting with the declarations of the personsToRemove and personsToUpdate lists, and the first three foreach loops (ie all but the one that displays the final result).

Notes:

• From the initialList , the code synthesizes an anonymous type containing the Person instance and the index of that instance in the list.
• The join clause pairs up all of the items from each list where the Value properties are equal.

Important: if there are multiple elements in either list with the same Value property, they are each paired with every other element in the other list having that same Value . Ie if initialList has two elements having the Value of "foo" and secondList has three such elements, you will wind up with six elements in the resulting join. Your question does not define whether this is possible, nor what you would want to happen if it were, so I've just ignored that possibility here. :)

• The join result is projected to a new anonymous type containing the index of the element to be replaced, and the new value.
• The query result is materialized by calling ToList() . This is necessary because the join is otherwise deferred and modifying the initialList would invalidate the query.
• Of course, in the remaining foreach all that the code then needs to do is assign to the appropriate index position in the list the replacement value determined by the query.

Answer 2

You can use Generics as well. Below is the short code will work for you:

    initialList.ForEach(p =>
    {
        if (secondList.Any(sp => sp.Value == p.Value))
        {
            initialList.Remove(p);
            initialList.Add(secondList.Single(spu => spu.Value == p.Value));
        };
    });