C# find all matching Items from a List

Question

Let\'s say we have a List and each product item in the list has many List

public class Product{
public int          


        

Answer 1

The following does what you want:

var productsPerType =
    from t in products.SelectMany(
        p => p.Types, (p, t) => new { Product = p, TypeId = t.Id })
    group t by t.TypeId
    into g
    select new { g.Key, Products = g.Select(x => x.Product) };

First, you do a SelectMany to get a list of all types inside the products. For each type you remember the type id and the corresponding product:

from t in products.SelectMany(
    p => p.Types, (p, t) => new { Product = p, TypeId = t.Id })

Each t is now an anonymous object containing a type id and a product. Next, you group these objects by type id. Now we have a group of products for each type id.

To give you an example, suppose you have the following products and types:

Product A -- Types 1, 2, 3
Product B -- Types 1
Product C -- Types 1, 3

The SelectMany gives the following intermediate result:

1, A
2, A
3, A
1, B
1, C
3, C

We group this result by type id so we get the following groups:

1, { A, B, C }
2, { A }
3, { A, C }

And this is the result you wanted.