How to get the list index of the nearest number?

Question

How to get the list index where you can find the closest number?

List<int> list = new List<int> { 2, 5, 7, 10 };
int number = 9;

int closest = list.Aggregate((x,y) => 
Math.Abs(x-number) < Math.Abs(y-number) ? x : y);

Answer 1

If you want the index of the closest number this will do the trick:

int index = list.IndexOf(closest);

Answer 2

You can include the index of each item in an anonymous class and pass it through to the aggregate function, and be available at the end of the query:

var closest = list
    .Select( (x, index) => new {Item = x, Index = index}) // Save the item index and item in an anonymous class
    .Aggregate((x,y) => Math.Abs(x.Item-number) < Math.Abs(y.Item-number) ? x : y);

var index = closest.Index;