var numbers = new[] {1,2,3,4};
var sumOfAllNumbers = numbers.Sum();
Console.WriteLine(sumOfAllNumbers); //10
var cities = new[] {
new {Population = 1000},
new {Population = 2500},
new {Population = 4000}
};
var totalPopulation = cities.Sum(c => c.Population);
Console.WriteLine(totalPopulation); //7500