CODEWITHSUNDEEP
c#lambda
alex
alex

Reputation: 23

C# Joins/Where with Linq and Lambda how to return a List<T>

The query below works just fine and returns one List<> exactly as expected

List<purchaseOrderHeaderEntity> details = (from p in 
db.purchaseOrderDetailEntity join r in db.purchaseOrderHeaderEntity on 
p.purchaseOrderID equals 
r.purchaseOrderID  
where p.productID == productID select r).ToList();

If I rewrite it using lambda, it returns two lists:

List<purchaseOrderHeaderEntity> lambda_details = db.purchaseOrderDetailEntity.Join(db.purchaseOrderHeaderEntity,
                    p => p.purchaseOrderID,
                    r => r.purchaseOrderID,
                    (p, r) => new { Detail = p, Header = r }).Where(DetailAndHeader => DetailAndHeader.Detail.productID == productID).ToList();

If I remove "Detail=p" from the selection part, Join throws an error

"The type arguments for method ... cannot be inferred from the usage. Try specifying the type arguments explicitly."

Is there a way to rewrite that query to make it return just one list for Header?

Thanks a lot in advance!

Upvotes: 1

Views: 272

Answers (2)

Eric Lippert
Eric Lippert

Reputation: 660024

Your query comprehension and your fluent query are different queries.

Your query comprehension expression is:

from p in db.purchaseOrderDetailEntity 
join r in db.purchaseOrderHeaderEntity on 
p.purchaseOrderID equals r.purchaseOrderID  
where p.productID == productID 
select r

Your fluent form is:

db.purchaseOrderDetailEntity.
  Join(
    db.purchaseOrderHeaderEntity,
    p => p.purchaseOrderID,
    r => r.purchaseOrderID,
    (p, r) => new { p, r }).
  Where(pr => pr.p.productID == productID)

But the correct fluent form is

db.purchaseOrderDetailEntity.
  Join(
    db.purchaseOrderHeaderEntity,
    p => p.purchaseOrderID,
    r => r.purchaseOrderID,
    (p, r) => new { p, r }).
  Where(pr => pr.p.productID == productID).
  Select(pr => pr.r)

Does that answer your question? If not, please give a small program that we can compile and run that clearly reproduces the problem.

Upvotes: 2

Lucian Bumb
Lucian Bumb

Reputation: 2881

you can put the where condition before join, then you can return the r. 

var lambda_details = db.purchaseOrderDetailEntity
             .Where(detailAndHeader => detailAndHeader.Detail.productID == productID)
             .Join(db.purchaseOrderHeaderEntity,
                        p => p.purchaseOrderID,
                        r => r.purchaseOrderID,
                        (p, r) => r).ToList();

Upvotes: 3

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