How do I use LINQ to join two collections in C#?

How do I use LINQ to join two collections in C#?

You can use the Join method in LINQ to join two collections. For example, if you have two lists of objects, you can join them based on a common key.

Can you show me a code example?

Sure! Here's a simple example:

var customers = new List<Customer> {
    new Customer { Id = 1, Name = "John" },
    new Customer { Id = 2, Name = "Jane" }
};

var orders = new List<Order> {
    new Order { Id = 1, CustomerId = 1, Product = "Laptop" },
    new Order { Id = 2, CustomerId = 2, Product = "Phone" }
};

var query = from c in customers
            join o in orders on c.Id equals o.CustomerId
            select new { c.Name, o.Product };

foreach (var item in query) {
    Console.WriteLine($"{item.Name} ordered {item.Product}");
}

What if I want to perform a left join instead?

For a left join, you can use the GroupJoin method followed by SelectMany. Here's an example:

var query = from c in customers
            join o in orders on c.Id equals o.CustomerId into customerOrders
            from co in customerOrders.DefaultIfEmpty()
            select new { c.Name, Product = co?.Product ?? "No Orders" };