删除对象C＃列表中的重复项

Question

I want to compare the element of a list of object ,delete the repeated Item and increment the number of the quantity of that Item (C# code ), I don't know if I should use LinQ,For or foreach statement : I have a list of OrderItem I want to delete the OrderItem that have the same FK_ArtikelId and increment the Qantity of the OrderItem . Exp:

for (int i=1 ; i < lot.Count   ; i ++)
{
    for (j = i + 1; j <= lot.Count; j++)
    {
        if (lot[i].FK_ArticleId.Equals(lot[j].FK_ArticleId))
        {
            lot[i].Quantity += lot[j].Quantity;
            lot.Remove(lot[j]);
        }
    }
}

Answer 1

You have to use the GroupBy linq method and process the resulting groups: given the class

public class Article
{
    public int FK_ArticleId { get; set; }
    public int Quantity { get; set; }
}

and the following list:

var list = new List<Article>()
{
    new Article() {FK_ArticleId = 1, Quantity = 10}
    , new Article() {FK_ArticleId = 1, Quantity = 10}
    , new Article() {FK_ArticleId = 1, Quantity = 10}
    , new Article() {FK_ArticleId = 2, Quantity = 100}
    , new Article() {FK_ArticleId = 2, Quantity = 100}
    , new Article() {FK_ArticleId = 3, Quantity = 1000}
};

The following linq query returns what you need:

list.GroupBy(a => a.FK_ArticleId)
    .Select(g => new Article() {FK_ArticleId = g.Key, Quantity = g.Sum(a => a.Quantity)});

// article id 1, quantity 30
// article id 2, quantity 200
// article id 3, quantity 1000

If you don't want to create a new article, you can take the first of the resulting group and set its Quantity to the correct value:

var results = list.GroupBy(a => a.FK_ArticleId)
    .Select(g =>
    {
        var firstArticleOfGroup = g.First();
        firstArticleOfGroup.Quantity = g.Sum(a => a.Quantity);
        return firstArticleOfGroup;
    });

Answer 2

I didn't test but this should give you an idea of the power of linq...

 var stuff  = lot
  .GroupBy(p => p.FK_ArticleId)
  .Select(g => g)
  .ToList();

This should give you groups of articleIDs whereby you can easily get counts, create new consolidated lists etc.

Answer 3

For starters you can't use foreach because you're modifying the list and the Enumerator will throw an exception. You can do the following with Linq:

var grouped = lot.GroupBy(x => x.FK_ArticleId).ToArray();
foreach(var group in grouped)
{
    group.First().Quantity = group.Sum(item => item.Quantity);
}

Now, first item in each group will contain the sum of all the quantities of items with the same FK_ArticleId . Now, to get the results use this:

var results = grouped.Select(g => g.First());

At this point it's purely your decision whether to return the results as a separate collection or insert them into the original list. If you opt for the second approach, don't forget to clear the list first:

list.Clear();
list.AddRange(results);

EDIT

A more elegant solution to accumulating the Quantity property into the first item of each group would be the following:

data.GroupBy(x=>x.FK_ArticleId)
    .Select(g => g.Aggregate((acc, item) => 
        {
             acc.Quantity = item.Quantity; 
             return acc;
        }));

This is what I scrapped in LinqPad: