IEnumerable variables in LINQ Queries in C#
When you see a query variable that is typed as IEnumerable<Customer>, it just means that the query, when it is executed, will produce a sequence of zero or more Customer objects.

Example: 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace ForgetCode
{   
    public class MainClass
    {
        public static void Main()
        {
            // Load your DB file here
            IEnumerable<Customer> customerQuery =
            from cust in customers
            where cust.City == "London"
            select cust;

            foreach (Customer customer in customerQuery)
            {
                Console.WriteLine(customer.LastName + ", " + customer.FirstName);
            }
        }
    }
}