Question

I am wondering why I have this problem with my list.

I am trying to retrieve the object for the previous index, so -1;

Is that the good way to do ?

thePreviousList.Find(previousItem => previousItem.Id - 1);

Answer 1

No, the best way is to use the index:

var item = thePreviousList[indexofItem];
var previousItem = thePreviousList[indexofItem -1];

But probably i’ve misunderstood your requirement.

If you want to find an item by it’s ID(assuming that it has this property):

var idToFind=4711;
var thisIndex = thePreviousList.Where((i, index) => i.ID==idToFind)
                   .Select((i, index) => index).FirstOrDefault();
if(thisIndex > 0)
{
    var previousItem = thePreviousList[thisIndex - 1];
}