public interface IDiscountStrategy
{
    decimal Apply(decimal total);
}

public sealed class RegularDiscount : IDiscountStrategy
{
    public decimal Apply(decimal total) => total;
}

public sealed class VipDiscount : IDiscountStrategy
{
    public decimal Apply(decimal total) => total * 0.9m;
}

public sealed class Checkout
{
    private readonly IDiscountStrategy _discount;

    public Checkout(IDiscountStrategy discount) => _discount = discount;

    public decimal TotalWithDiscount(decimal total) => _discount.Apply(total);
}

var checkout = serviceProvider.GetRequiredService<Checkout>();
var total = checkout.TotalWithDiscount(100m);

var totals = orders
    .GroupBy(o => o.CustomerId)
    .Select(g => new
    {
        CustomerId = g.Key,
        Total = g.Sum(o => o.Amount)
    })
    .ToList();

var totals = new Dictionary<int, decimal>();
foreach (var order in orders)
{
    if (totals.TryGetValue(order.CustomerId, out var current))
        totals[order.CustomerId] = current + order.Amount;
    else
        totals[order.CustomerId] = order.Amount;
}

var youngest = people
    .OrderBy(p => p.Age)
    .First();

var youngest = people.MinBy(p => p.Age);