LEFT OUTER JOIN in LINQ

Question

How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause? Corr

Answer 1

I would like to add that if you get the MoreLinq extension there is now support for both homogenous and heterogeneous left joins now

http://morelinq.github.io/2.8/ref/api/html/Overload_MoreLinq_MoreEnumerable_LeftJoin.htm

example:

//Pretend a ClientCompany object and an Employee object both have a ClientCompanyID key on them

return DataContext.ClientCompany
    .LeftJoin(DataContext.Employees,                         //Table being joined
        company => company.ClientCompanyID,                  //First key
        employee => employee.ClientCompanyID,                //Second Key
        company => new {company, employee = (Employee)null}, //Result selector when there isn't a match
        (company, employee) => new { company, employee });   //Result selector when there is a match

EDIT:

In retrospect this may work, but it converts the IQueryable to an IEnumerable as morelinq does not convert the query to SQL.

You can instead use a GroupJoin as described here: https://stackoverflow.com/a/24273804/4251433

This will ensure that it stays as an IQueryable in case you need to do further logical operations on it later.