如何展平2层对象列表并总结其他属性常见的属性

Question

I realise the name of this question is confusing, so let me rather explain my scenario.

I have a list of objects. This list has an entry for each user. Each user then has a list of categories, and each category has a category id, and some values that have already been calculated. I need to flatten my objects in a list of categories, where the properties of each category has been summed. Each user will have the same amount of categories in their lists, and each category will be present in each user's lists (in other words, each user will have the same list of categories, the calculated properties will only be different).

My classes are:

public class category {
     public int categoryID { get; set; }
     public decimal categoryValue { get; set; }
     public decimal categoryValue2 { get; set; }
}

public class user {
    public List<category> categories { get; set; }
}

What I need to achieve is by using Linq (or whatever means will be most efficient) to flatten the data to a single List where the categories are still split up by their respective ID's but categoryValue and categoryValue2 have been totaled respectively. Hence I get a 2 totaled value per category for all users.

I have tried doing a for loop as follows:

List<category> categoryTotals = new List<category> {};
foreach (category loopCategory in users[0].categories) {
    category currentCategory = new category();
    category = users.Select(x => x.categories).Where(y=>y.categoryID== loopCategory.categoryID)).Select(t => new {t....
}

But the t is giving me linq functions, not the properties that I want to sum.

I have also tried a similar approach using just linq, but after doing the SelectMany, followed by the GroupBy, the succeeding select statement again doesn't expose the category lists's properties.

Many thanks!

Answer 1

Is this what you want?

var groupSums = 
    from g in users.SelectMany(u => u.categories).GroupBy(c => c.categoryID)
    select new
    {
        categoryID = g.Key,
        CategoryValueSum = g.Sum(c => c.categoryValue),
        CategoryValue2Sum = g.Sum(c => c.categoryValue2),
    };

users.SelectMany(u => u.categories).GroupBy(c => c.categoryID) gives you a sequence of Group<category> objects. Each has a Key property equal to the categoryID for the group, and also implements IEnumerable<category> .

The whole thing gives you a flat sequence of anonymous objects with a categoryID property, and two sum properties for the sums of the two category values. Anonymous classes can be limiting, so depending on what you're doing with the results, you may want to write a new quickie class for that.

And of course if you need to keep the results around for some reason, call ToList() on the results.

Answer 2

You can use LINQ SelectMany method combined woth GroupBy

var sumList = users
    .SelectMany(u => u.categories)
    .GroupBy(c => c.categoryID)
    .Select(g => new category()
    {
       categoryID = g.Key,
       categoryValue = g.Sum(z => z.categoryValue),
       categoryValue2 = g.Sum(z => z.categoryValue2),
    })
   .ToList();

This will return List<category> with categoryValue and categoryValue2 summed for all users.

Answer 3

tested it out and it looks like this. You will get grouped category list with your desired aggregated result.

    var cats = users.SelectMany(x => x.categories).GroupBy(x => x.CategoryID)
        .Select(x => new Category()
        {
            CategoryID = x.Key,
            CategoryValue = x.Sum(y => y.CategoryValue),
            CategoryValue2 = x.Sum(y => y.CategoryValue2)
        });