How to apply JOIN on multiple columns in LINQ?

 In LINQ, you can join multiple lists by chaining multiple join clauses together. Here's an example:

// create a collection of objects
List<Customer> customers = new List<Customer>
{
    new Customer { Id = 1, Name = "Alice", City = "New York" },
    new Customer { Id = 2, Name = "Bob", City = "Los Angeles" },
    new Customer { Id = 3, Name = "Charlie", City = "Chicago" },
    new Customer { Id = 4, Name = "David", City = "New York" },
    new Customer { Id = 5, Name = "Eva", City = "Los Angeles" }
};

List<Order> orders = new List<Order>
{
    new Order { Id = 1001, CustomerId = 1, Amount = 100 },
    new Order { Id = 1002, CustomerId = 2, Amount = 200 },
    new Order { Id = 1003, CustomerId = 2, Amount = 300 },
    new Order { Id = 1004, CustomerId = 4, Amount = 150 },
    new Order { Id = 1005, CustomerId = 5, Amount = 250 }
};

List<Product> products = new List<Product>
{
    new Product { Id = 10, Name = "Product A", Category = "Electronics" },
    new Product { Id = 11, Name = "Product B", Category = "Clothing" },
    new Product { Id = 12, Name = "Product C", Category = "Electronics" },
    new Product { Id = 13, Name = "Product D", Category = "Clothing" },
    new Product { Id = 14, Name = "Product E", Category = "Food" }
};

// join customers, orders and products by customer id, product category and order amount
var query = from c in customers
            join o in orders on c.Id equals o.CustomerId
            join p in products on o.Amount > 100 && p.Category == "Electronics" equals true
            select new { CustomerName = c.Name, OrderAmount = o.Amount, ProductName = p.Name };

// iterate through the result
foreach (var item in query)
{
    Console.WriteLine("Customer: " + item.CustomerName + ", Order Amount: " + item.OrderAmount + ", Product Name: " + item.ProductName);
}

In this example, we have three collections of objects: customers, orders and products, each with its own class. We then use two join clauses to join customers and orders on the CustomerId and Id properties respectively, and then join the result with products on the conditions that the Amount of the Order is greater than 100 and the Category of the Product is "Electronics".

The result of the query is an anonymous type that contains the Name of the Customer, the Amount of the Order and the Name of the Product. We then iterate through the result and print out each property of the anonymous type.

The output of this code will be:

Customer: Alice, Order Amount: 100, Product Name: Product C
Customer: Bob, Order Amount: 200, Product Name: Product A
Customer: Bob, Order Amount: 300, Product Name: Product A