Use Linq and Lambda to include default

Question

I have two lists: relatedContact and fileContactIds. I am using the query to loop the relatedContacts list and return value of userClientCode (which I will make the image name) only if it is also in the fileContactIds list. How can I put in a value (eg: "Default") if the value isn't in the second list ?

var result = relatedContact.SelectMany(rc => rc.contacts.Select(pc => new RelatedContactsDescription
                {
                    imageUrl = mappedRelatedContactsPath + pc.userclientcode + ".jpg",
                    userclientcode = pc.userclientcode,
                    description = rc.clienttaxonomy,
                    fullname = pc.fullname,
                    email = pc.contactdetails != null && pc.contactdetails.Count >= 1 ? pc.contactdetails[0].contactdata : "",
                    address = pc.contactdetails != null && pc.contactdetails.Count >= 2 ? pc.contactdetails[1].contactdata : "",
                    phoneNumber = pc.contactdetails != null && pc.contactdetails.Count >= 3 ? pc.contactdetails[2].contactdata : "",
                    populated = string.IsNullOrEmpty(pc.userclientcode) ||
                                string.IsNullOrEmpty(pc.fullname) ||
                                string.IsNullOrEmpty(pc.contactdetails[0].contactdata) ? false : true,
                }))
                .Where(el => fileContactIds.Contains(el.userclientcode)).ToList();

I have read: Lambdas and Linq and Joins First or Default etc but all I can see is how to get matching data from both not also how to put a "fall-back" or default value if they don't match.

Thanks in advance

Btw which column needs Default Value, userclientcode is it ? — Mrinal Kamboj, Nov 12 '16 at 06:18
Not related but why do you combine everything into 1 line? (`select`, `selectmany`, `where`, `tolist`, `count`, `sisnullorempty`, `count`, `if-else`...)? — Tân, Nov 12 '16 at 06:35
@vipin Check my answer underneath, it contains two options, one using Left Join, which is achieved using `GroupJoin` — Mrinal Kamboj, Nov 12 '16 at 08:54

Mrinal Kamboj · Accepted Answer · 2016-11-12T08:53:20.223

3

In your current code do the following changes:

Remove the Where clause, as that's not serving the purpose, you need Default where UserClientCode doesn't exist, you don't want to filter them out using Where
Change the following in SelectMany call:

userclientcode = pc.userclientcode

To

 userclientcode = fileContactIds.Contains(pc.userclientcode) ? 
                  pc.userclientcode : "Default"

You may consider using a Left Join post creating a flattened list using SelectMany, but that would be a Round about way to achieve it, which is simply achieve while creating a flattened List.

Using Left Join

Following is the code option for Left Join, it needs GroupJoin in C#:

var result = relatedContact.SelectMany(rc => rc.contacts.Select(pc => {pc})
                           .GroupJoin(fileContactIds, pc => pc.userclientcode,
                                      fc => fc.ContactId,new {pc,fc})
                           .SelectMany(
                            x => x.fc.DefaultIfEmpty()
                            (x,y) => new RelatedContactsDescription
                            {
                               userclientcode = (y == null) ? "Default":x.pc.userclientcode,
                            .........(fill remaining as per original logic)
                            });

How does it work

Create Anonymous List from relatedContact using SelectMany
GroupJoin with fileContactIds. where I have assume field is ContactId
Create an Anonymous List with Data from both collections, where Unmatched records from fileContactIds will be empty being LeftJoin
In final SelectManyexplicity make the fileContactIds empty values null, then during data selection, so what I have suggested earlier for null values, fill "Default", else the specific value

edited Nov 12 '16 at 08:53

answered Nov 12 '16 at 06:21

Mrinal Kamboj

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I still think He should use left join – Inside Man Nov 12 '16 at 07:18
What I have suggested is much simpler and would be done while creating the final list. As I have suggested, yes `Join` is an option but its the matter of simplicity in my view – Mrinal Kamboj Nov 12 '16 at 07:59
@Stranger check out the code for Left Join too, which is achieved using `GroupJoin` – Mrinal Kamboj Nov 12 '16 at 08:45
@Stranger Also Check I have explained its working, this is good example: http://www.advancesharp.com/blog/1108/linq-inner-join-left-outer-join-on-two-lists-in-c-with-example – Mrinal Kamboj Nov 12 '16 at 08:53
@MrinalKamboj I am trying to implement your first suggestion but on getting what you mean when you say remove Where clause. Do you mean after Select I should then filter ? Also your left join is showing many errors and extra brackets are inserted. Also please remember I don't just want the userclientcode, I want to match this to the contactId and map into RelatedContactsDescription object – Afshin Ghazi Nov 14 '16 at 07:55
@AfshinGhazi, its like, `Where` clause in your code is only for the purpose of filtering the `Ids` based on `fileContactIds`, but your challenge is you want Default value for Ids which doesn't exist, not just filter them out, so I have shifted the filtering inside the `SelectMany` and `Where` clause is redundant as it cannot help you to get the Default values. – Mrinal Kamboj Nov 14 '16 at 08:09

Use Linq and Lambda to include default

1 Answers1