Why doesn't this LINQ Select expression work

Question

I've got a difficult LINQ expression and I can't figure out why it doesn't work. The syntax error I get is

The type of arguments for Enumerable.Select<TSource, TResult>(IEnumerable<TSource>, Func<TSource, TResult>) cannot be inferred from the usage. Try specifying the type arguments explicitly.

The error is on the second Select statement, x.Select . I'm trying to grab one element in each List of List from allFactors and add them together and keep each of each grouping that was added together in tempList . In other words, I want to keep the individual elements together in tempList and know their total in temp .

Earlier in the code allFactors is filled with values. How can I specify the types explicitly or do this another way. I can't understand why it doesn't infer the types either.

int temp = 0;
//List<List<int>> allFactors = new List<List<int>>();
List<int> tempList = new List<int>();
allFactors.Select(x => x.Select(y => { temp += y; tempList.Add(y); }));

EDIT: David L's answer does fix the syntax error! Unfortunately with further testing I realize that my code isn't doing what I wanted it to do. What I really want is to get every permutation whereby each group is made of only one element from a list of lists. As an example:

List<List<int>> oldList = {{1,2},{3,4}};
List<List<int>> newList = {{1,3},{1,4},{2,3},{2,4}};

I'm looking for some way to convert oldList into newList . The challenge is that I don't know how many nested lists there will be or how many items will be in each list. Any ideas? Thanks for everyone's thoughts so far.

Answer 1

The type cannot be inferred because you are not returning anything by the inner select. The compiler doesn't have anything to infer for the outer select as a result.

In addition, since you are not using the selected return, you could use .ForEach() instead.

int temp = 0;
List<List<int>> allFactors = new List<List<int>>();
List<int> tempList = new List<int>();
allFactors.ForEach(x => x.ForEach(y => { temp += y; tempList.Add(y); }));

---

If you wanted to stick with .Select() , you would need to return the value from the inner select and use .SelectMany() for the outer select.

int temp = 0;
List<List<int>> allFactors = new List<List<int>>();
List<int> tempList = new List<int>();
List<int> selectedList = allFactors.SelectMany(x => x.Select(y => 
                { 
                    temp += y;    
                    tempList.Add(y); 
                    return y; 
                })).ToList();

This would produce a "flattened" List<int> , which seems to be in line with your end goal for tempList .

Answer 2

If you want just flatten the "allFactors" you can go this way:

        var tempList = allFactors.SelectMany(x => x).ToList();
        var temp = tempList.Sum();

If you need only first element of each list, then it will be:

        var tempList = allFactors.Select(x => x.First()).ToList();
        var temp = tempList.Sum();