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

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.

CodePudding user response：

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();