List.Clear（）无法正常工作

Question

private Map GetMinMaxDistanceShcoolData(Map MonitorsData)
{
    var tempData = MonitorsData.MonitorsDataList;

    if (tempData != null)
    {
        foreach (var monitor in tempData)
        {
            foreach (var tehsil in monitor.Tehsils)
            {
                var ordered = tehsil.Schools.OrderBy(x => x.Distance).ToList();
                var min = ordered.FirstOrDefault();
                var max = ordered.LastOrDefault();

                ordered.ToList().Clear();

                tehsil.Schools.ToList().Add(min);
                tehsil.Schools.ToList().Add(max);
            }
        }
    }

    return null;
}

Before and After clearing the list the count of the list is same. Even when I am using the Remove() instead of Clear(), it is also not removing the list. Also the Add(min) and Add(max) is not working.

Answer 1

It's working just fine, you're just calling one many ToList

ordered is a List, then you call ToList (creating a new List), then you clear that last List (not touching the first one).

What you wrote :

ordered.ToList().Clear();

Is the same as

var newlist = ordered.ToList();
newlist.Clear(); // of course this doesn't clear ordered

What you need to do is simply

ordered.Clear();

Answer 2

You are doing the wrong thing. ToList creates a new List, therefore Ordered is a new list, and you clear that, but not the orginal data.

 ordered.ToList().Clear();

Also ordered is already a list, calling ToList is redundant

I think you meant to do this:

var ordered = tehsil.Schools.OrderBy(x => x.Distance).ToList();
var min = ordered.FirstOrDefault();
var max = ordered.LastOrDefault();

tehsil.Schools.Clear();

tehsil.Schools.Add(min);
tehsil.Schools.Add(max);

Answer 3

Remove the ToList() call before Clear()

var tempData = MonitorsData.MonitorsDataList;

if (tempData != null)
{
    foreach (var monitor in tempData)
    {
        foreach (var tehsil in monitor.Tehsils)
        {
            var ordered = tehsil.Schools.OrderBy(x => .Distance).ToList();
            var min = ordered.FirstOrDefault();
            var max = ordered.LastOrDefault();

            ordered.Clear();

            tehsil.Schools.ToList().Add(min);
            tehsil.Schools.ToList().Add(max);
        }
    }
}

Answer 4

Replace ordered.ToList().Clear(); with ordered.Clear();

Answer 5

You're clearing the list created by .ToList()

ordered.ToList().Clear();
// .ToList() - Creates a new list.
// .Clear() - Clears the list that was created.

To clear a list use:

ordered.Clear();
// .Clear() - Clears the list called ordered.

Answer 6

hi you have a few issues here, try something like this.

foreach (var tehsil in monitor.Tehsils)
{
    var ordered = tehsil.Schools.OrderBy(x => x.Distance).ToList();
    if(!ordered.Any || ordered.Count < 3) continue;
    var maxmin = new List<tehsil.Schools> { ordered.FirstOrDefault(), ordered.LastOrDefault() }
    tehsil.Schools = maxmin;
}