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How to select existing objects in LINQ query

I have a table Companies

CompanyId
CountryId
and so on...


And then have a list of objects with following structure (let's name this list dataset)

Id
CountryId
CurrencyId
TotalCost
and so on...


and what I'm trying to do is to get all companies which exists in above dataset,
so I'm trying to write query like:

IEnumerable<Company> companies = db.Companies.Where(a=a.CompanyId == dataset.Select(a=>a.CompanyId).ToList()


but it gives me an error,

Operator '==' cannot be applied to operands of typ int and List

How to get rid of this error?

Where(a=a.CompanyId == dataset.Select(a=>a.CompanyId)

Where(a => a.CompanyId == companyId

companyId = dataset.Select(a=>a.CompanyId).FirstOrDefault()

myVar = someInt

int

List

4 Answers
4

IEnumerable<Company> companies = db.Companies.Where(a=> dataset.Select(b=>b.CompanyId).Contains(a.CompanyId)).ToList()


Try this , you are trying to use list of int == on single int variable .. use contains to do that

List<int> companyIds = dataset.Select(a=>a.CompanyId).ToList();
IEnumerable<Company> companies = db.Companies.Where(a=> companyIds.Contains(a.CompanyId));


You can pre-calculate the company ids to be searched and then use Contains method of List<T> to match the company ids.

Contains

List<T>

You could use Contains() like this:

Contains()

var companies = new List<(int companyid, int countryid, string companyName)>()

(1, 2, "One Incorporated"),
(2, 5, "Two-two Ltd"),
(3, 24, "Mr-Three Ltd")
;

var other = new List<(int companyid, string stuff)>()

(1, "asdad"),
(2, "asdadv")
;

var inOther = companies.Where(x => other.Select(y => y.companyid).Contains(x.companyid));


This would output "One Incorporated" and "Two-two Ltd".

other.Select(y => y.companyid) gets your list of ID's then Contains() checks to see if x's (x being the company in the where statement) ID exists.

dataset.Select(a=>a.CompanyId) return a List, so you can't compare your simple CompanyId type int with this List

dataset.Select(a=>a.CompanyId)

List

CompanyId

int

List

You must check if an object with his CompanyId being in this list allowed or not by the method Contains

CompanyId

Contains

var companies = db.Companies.Where(a=> dataset.Select(a=>a.CompanyId).Contains(a.CompanyId))


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