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 combine two collections based on a common key. For example, if you have two lists, one of customers and one of orders, you can join them on the customer ID.

Can you show me a code example of that?

Sure! Here's a simple example:

var customers = new List<Customer> { ... };
var orders = new List<Order> { ... };

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

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 how:

var query = from c in customers
            join o in orders on c.CustomerId equals o.CustomerId into customerOrders
            from co in customerOrders.DefaultIfEmpty()
            select new { c.Name, OrderDate = co?.OrderDate };