Finding common elements within a list of objects

Question

I have the following code

public class OrderDetails
{
    public int OrderId { get; set; }

    public int CustomerId { get; set; }

}

public List<int> GetCustomerIds()
{
    var orderDetails = new List<OrderDetails>
    {
        new OrderDetails {OrderId = 1001, CustomerId = 2001},
        new OrderDetails {OrderId = 1001, CustomerId = 2002},

        new OrderDetails {OrderId = 1002, CustomerId = 2003},
        new OrderDetails {OrderId = 1002, CustomerId = 2002},

        new OrderDetails {OrderId = 1003, CustomerId = 2003},
        new OrderDetails {OrderId = 1003, CustomerId = 2002},     
        //return list of common customerIds.           
    };
} 

I want to get a list of CustomerIds that are common across all orders. In the example above my output would be 2002. What is a clean way of achieving this?

Answer 1

This will accomplish what you want:

var result = orders.GroupBy(x => x.CustomerId)
                   .Where(y => y.Count() == orderDetails.GroupBy(z => z.OrderId)
                                                        .Count())
                   .FirstOrDefault()
                   .Key;

Output:

2002

We can read the purpose from the query in English:

• Group your collection on the CustomerId
• Gather how many different orders there are by grouping on OrderId and getting the Count()
• Filter the collection to those customers which appear as often as there are orders
• Select the first
• Print the key (which is the CustomerId)

Important note: this will only take the first customer in case there are multiple with the same amount of orders, but changing this to take them all is trivial.