LINQ Join with Multiple Conditions in On Clause

LINQ Join with Multiple Conditions in On Clause,linq,join,Linq,Join,I'm trying to implement a query in LINQ that uses a left outer join with multiple conditions in the ON clause. I'll use the example of the following two tables Project (ProjectID, ProjectName) and Task (TaskID, ProjectID, TaskName, Completed). I want to see the full list of all projects with their respective tasks, but only those tasks that are completed. I cannot use a filter for Completed == true because that will filter out any projects that do not have completed tasks. Instead I want to ad

I'm trying to implement a query in LINQ that uses a left outer join with multiple conditions in the ON clause.

I'll use the example of the following two tables Project (ProjectID, ProjectName) and Task (TaskID, ProjectID, TaskName, Completed). I want to see the full list of all projects with their respective tasks, but only those tasks that are completed.

I cannot use a filter for

Completed == true
because that will filter out any projects that do not have completed tasks. Instead I want to add
Completed == true
to the ON clause of the join so that the full list of projects will be shown, but only completed tasks will be shown. Projects with no completed tasks will show a single row with a null value for Task.

Here's the foundation of the query.

from t1 in Projects
join t2 in Tasks
on new { t1.ProjectID} equals new { t2.ProjectID } into j1
from j2 in j1.DefaultIfEmpty()
select new { t1.ProjectName, t2.TaskName }

How do I add

&& t2.Completed == true
to the on clause?

I can't seem to find any LINQ documentation on how to do this.


#1

You just need to name the anonymous property the same on both sides

on new { t1.ProjectID, SecondProperty = true } equals 
   new { t2.ProjectID, SecondProperty = t2.Completed } into j1

Based on the comments of @svick, here is another implementation that might make more sense:

from t1 in Projects
from t2 in Tasks.Where(x => t1.ProjectID == x.ProjectID && x.Completed == true)
                .DefaultIfEmpty()
select new { t1.ProjectName, t2.TaskName }

#2

Here you go with:

from b in _dbContext.Burden 
join bl in _dbContext.BurdenLookups on
new { Organization_Type = b.Organization_Type_ID, Cost_Type = b.Cost_Type_ID } equals
new { Organization_Type = bl.Organization_Type_ID, Cost_Type = bl.Cost_Type_ID }

#3

You can't do it like that. The join clause (and the Join() extension method) supports only equijoins. That's also the reason, why it uses equals and not ==. And even if you could do something like that, it wouldn't work, because join is an inner join, not outer join.


#4

Related answer here using Lambda syntax

#5

That seems like a non obvious way to do that. I'm not sure I would understand what it's suppose to do.

#6

@svick - Using anonymous types allow you to join on multiple criteria. You just need to make sure the property names match on both types. Not sure where the confusion is coming from?

#7

The confusion is that it really makes more sense as two equalities joined by and, not one equality of some “weird” object. And to prove my point, your code is wrong. For it work, you would have to have true on the left side and t2.Complete on the right.

#8

Thanks Aducci. I had to swap sides in the query to get the context right, but that worked. This problem is simplified, and in my real world problem it's not just SecondProperty is true or false, SecondProperty is an integer and I use AND SecondProperty IN (123, 456). I'll be moving on to that challenge and any help you could give would be greatly appreciated.

#9

@svick - Good catch, I switched the order of the t2.Completed and the true value. I added another solution that might be less weird for you.

#10

This looks more understandable.

#11

Outer join was not requested, and (see other answers), obviously you can.