LINQ – Left Join Example in C#

In this post, we will see an example of how to do a Left Outer Join in LINQ and C#.

In a previous post, we saw how to do an Inner join in C# and LINQ where each element of the first collection appears one time for every matching element in the second collection. If an element in the first collection has no matching elements, it does not appear in the join result set. However in a Left Outer Join, each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection.

Let us see this with an example.

class Program{
    static void Main(string[] args)
    {
        List<Book> bookList = new List<Book>
        {
            new Book{BookID=1, BookNm="DevCurry.com Developer Tips"},
            new Book{BookID=2, BookNm=".NET and COM for Newbies"},
            new Book{BookID=3, BookNm="51 jQuery ASP.NET Recipes"},
            new Book{BookID=4, BookNm="Motivational Gurus"},
            new Book{BookID=5, BookNm="Spiritual Gurus"}
        };

        List<Order> bookOrders = new List<Order>{
            new Order{OrderID=1, BookID=1, PaymentMode="Cheque"},
            new Order{OrderID=2, BookID=5, PaymentMode="Credit"},
            new Order{OrderID=3, BookID=1, PaymentMode="Cash"},
            new Order{OrderID=4, BookID=3, PaymentMode="Cheque"},
            new Order{OrderID=5, BookID=5, PaymentMode="Cheque"},
            new Order{OrderID=6, BookID=4, PaymentMode="Cash"}
        };
    }
}

public class Book{
    public int BookID { get; set; }
    public string BookNm { get; set; }
}

public class Order{
    public int OrderID { get; set; }
    public int BookID { get; set; }
    public string PaymentMode { get; set; }
}
}

Let us do a Left Outer Join between the Book and Order collection

var orderForBooks = from bk in bookList
            join ordr in bookOrders
            on bk.BookID equals ordr.BookID
            into a
            from b in a.DefaultIfEmpty(new Order())
            select new
            {
                bk.BookID,
                Name = bk.BookNm,
                b.PaymentMode
            };

foreach (var item in orderForBooks)
    Console.WriteLine(item);

Console.ReadLine();

In the code shown above, the query uses the join clause to match Book objects with Order objects testing it for equality using the equals operator. Up till here, the query is the same as in our previous article.

Additionally in order to include each element of the Book collection in the result set even if that element has no matches in the Order collection, we are using DefaultIfEmpty() and passing in an empty instance of the Order class, when there is no Order for that Book.

The select clause defines how the result will appear using anonymous types that consist of the BookID, Book Name and Order Payment Mode.

OUTPUT

Observe that BookID =2 was included in the list even though it did not have an entry in the Order table. You can compare this result with the one we got in our previous article to understand the difference between Inner Join and Left Outer Join.