Say there is a pair model that has a FirstId and a SecondId Guid property. I want to create a list of ids. Right now I am doing it like this:
var ids = new List<Guid> { };
payload.Pairs
.ToList()
.ForEach(x =>
{
ids.Add(x.FirstId);
ids.Add(x.SecondId);
});
Is there some magic method within linq that can do that instead of ForEach?
Use SelectMany:
ids = payload.Pairs.SelectMany(p => new[]{ p.FirstId, p.SecondId }).ToList();
Your approach could be translated to Linq's Aggregate function
var ids = payload.Pairs.Aggregate(
seed: new List<Guid>(),
func: (agg, item) => {
agg.AddRange(new[] { item.FirstId, item.SecondId });
return agg;
},
resultSelector: agg => agg);
If your payload.Pairs is already materialized and it is not an IEnumerable then you can do some memory optimisation like this
var ids = payload.Pairs.Aggregate(
seed: new List<Guid>(payload.Pairs.Count * 2),
func: (agg, item) => {
agg.Add(item.FirstId);
agg.Add(item.SecondId);
return agg;
},
resultSelector: agg => agg);
Yet another type of solution could be to use two stream of guids
var firstIds = payload.Pairs.Select(item => item.FirstId);
var secondIds = payload.Pairs.Select(item => item.SecondId);
and then depending on the fact the ordering matters or not you can use Zip or Concat respectively.
firstIds.Concat(secondIds);
//OR
firstIds.Zip(secondIds, (f, s) => new[] { f, s }).SelectMany(x => x);
Alongside SelectMany, if you're looking to add them to an existing list, the AddRange function is the best to use:
ids.AddRange(payload.Pairs.SelectMany(p => new[] { p.FirstId, p.SecondId }));
