Return values from list that are not present in another list using LinQ

November 22, 2022

Hey i have list of integers which are id’s.
I want to create async method that returns list of id’s that are not present in dbContext table. I have something like this.

public async Task<List<int>> NotExisting(List<int> inputIds, CancellationToken cancellationtoken)
{
var notExisting = await _dbContext.Table
    .Where(x => !inputIds.Contains(x.Id)).Select(x => x.Id).ToListAsync();

return notExisting;
}

Now its just taking all of the elements in table that doesnt match the input id. I want to only get the elements from inputIds that are not present in _dbContext.Table. I know i am taking wrong thing in Select but i dont know how to access the current id in inputIds.

>Solution :

You just need to switch the SELECT source, since the IDs you want to have are ultimately NOT coming from the database:

var allIDsInDb = (await _dbContext.Table.Select(x => x.Id).ToListAsync()).ToHashSet();
// ToHashSet() was for faster lookups in the next line
var notExisting = inputIds.Except(allIDsInDb).ToList();

UPD: If you don’t want to pull all the IDs from the database, you still need to find out which IDs from your list DO exist in the db, so just add a condition:

var allIDsInDb = (await _dbContext.Table.Where(x => inputIds.Contains(x.Id)).Select(x => x.Id).ToListAsync()).ToHashSet();
// ToHashSet() was for faster lookups in the next line
var notExisting = inputIds.Except(allIDsInDb).ToList();