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=" 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
Name = bk.BookNm,

foreach (var item in orderForBooks)


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.



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.

Make sure you read my previous article Inner Join Example in LINQ and C# to understand the difference between the Inner Join and Left Outer Join.

About The Author

Suprotim Agarwal
Suprotim Agarwal, Developer Technologies MVP (Microsoft Most Valuable Professional) is the founder and contributor for DevCurry, DotNetCurry and SQLServerCurry. He is the Chief Editor of a Developer Magazine called DNC Magazine. He has also authored two Books - 51 Recipes using jQuery with ASP.NET Controls. and The Absolutely Awesome jQuery CookBook.

Follow him on twitter @suprotimagarwal.


Dan Miser said...

Nice post. That syntax shows how to simulate a left outer join when the 2 tables aren't related, but in things like LINQ to SQL, you can just use this:

var orderForBooks =
from bk in Books
select new
Name = bk.Name,
PaymentMode =
from o in bk.Orders
select o.PaymentMode

Granted, the shape of the result set is different (not flattened one per row), but it's another option. You just need to be sure to set up FKs in your database.

Suprotim Agarwal said...

Yes! Thanks for sharing your code Dan.

Gediminas said...

Dan Miser's solution creates structure very different from SQL Left join. PaymentMode is IEnumerable collection packed anonymous object. It may be empty. Using this structure in pure C# code is not a problem but sending it to Reporting Services requires additional transformations

Anonymous said...

It would be nice if your example was for a left outer join on a DB table rather than two hard coded objects.

Golem said...

Nice Code, but to specify each field in "select" is very "static" I would say.

My problem is, that do not know the data structure i'am going to join, because it will be specified by user interactions in runtime.

I want to join two "EnumerableRowCollection"s with unknown fields, but linq allows me only to "select" one of the entity structures at the end.

Actually I expected same results like a "SQL-Left-Join", that will join table structures as well...

My code:

var baseTable = GetDataTable(...);
var joinTable = GetDataTable(...);

var baseTableAsEnumerable = baseTable.AsEnumerable();
var joinTableAsEnumerable = joinTable.AsEnumerable();

var joinResult = from baseAlias in baseTableAsEnumerable
join joinAlias in joinTableAsEnumerable
on baseAlias.Field(baseColumn)
equals joinAlias.Field(joinColumn)
into joinedQueryResult
select joinedQueryResult.First();

The result of this example only contains column structure of "joinTableAsEnumerable", but should contain both - "baseTableAsEnumerable" AND "joinTableAsEnumerable".

My idea was to "create" a new "DataRow" by my own, but this will not result in a correct join-entity at the end. Take a look:

into joinedQueryResult
select GetNewJoinedRow(baseTable.Columns, joinTable.Columns, joinedQueryResult.First());

Any solutions?