LINQ provides methods to group and aggregate data based on specific properties or conditions.

GroupBy

The GroupBy method allows you to group elements in a collection based on a key selector function. The result is a collection of groups, each containing elements with the same key value.

Example:

List<Product> products = GetProducts();

var groupedProducts = products.GroupBy(p => p.Category);

foreach (var group in groupedProducts)
{
    Console.WriteLine($"Category: {group.Key}");
    foreach (var product in group)
    {
        Console.WriteLine($"  Product: {product.Name}");
    }
}

GroupJoin

The GroupJoin method allows you to perform a grouped join between two collections based on a key selector function for each collection.

Example:

List<Customer> customers = GetCustomers();
List<Order> orders = GetOrders();

var customerOrders = customers.GroupJoin(orders,
    customer => customer.Id,
    order => order.CustomerId,
    (customer, orderGroup) => new
    {
        Customer = customer,
        Orders = orderGroup
    });

foreach (var item in customerOrders)
{
    Console.WriteLine($"Customer: {item.Customer.Name}");
    foreach (var order in item.Orders)
    {
        Console.WriteLine($"  Order: {order.Id}");
    }
}