check if record is last or first in list with linq

Question

I have a list of objects. I want to determine when the user will get the first or last object in the list, that way I can disable some buttons on the page.

For example I might have some boolean and if the object requested is last or first in the list, then it will return true, otherwise false.

Any idea?

Solution 1

You can wrtie an extension method such as

public static class IEnumerableExtensions
{
    public static bool IsLast<T>(this IEnumerable<T> items, T item)
    {
        var last =  items.LastOrDefault();
        if (last == null)
            return false;
        return item.Equals(last); // OR Object.ReferenceEquals(last, item)
    }

    public static bool IsFirst<T>(this IEnumerable<T> items, T item)
    {
        var first =  items.FirstOrDefault();
        if (first == null)
            return false;
        return item.Equals(first);
    }

    public static bool IsFirstOrLast<T>(this IEnumerable<T> items, T item)
    {
        return items.IsFirst(item) || items.IsLast(item);
    }
 }

You can use it like

 IEnumerable<User> users = // something
 User user = // something

 bool isFirstOrLast = users.IsFirstOrLast(user);

Solution 2

If your list of objects is indeed a List, it might be better to use it explicitly (adapted the Mehmet Ataş's answer):

static class ListExtensions
{
  public static bool IsFirst<T>(this List<T> items, T item)
  {
    if (items.Count == 0)
      return false;
    T first =  items[0];
    return item.Equals(first);
  }

  public static bool IsLast<T>(this List<T> items, T item)
  {
    if (items.Count == 0)
      return false;
    T last =  items[items.Count-1];
    return item.Equals(last);
  }
}

This way you eliminate the LINQ overhead (it's not much, but it's significant). However, your code must use List<T> for this to work.

Solution 3

var yourObject = yourList[0];
if(list.Count > 0)
    if(yourObject == list.First() || yourobject == list.Last())
    {
     //item is either first or last
    }

But remember to check if the list contains atleast 1 item, otherwise you will end up with exception from First, Last.

The above will compare the reference for the objects, you can compare their values by implementing a custom IComparable or you may compare their values.

Comments

[spender]

...also worth being aware that in most/all cases, .Last() causes a full enumeration of the source sequence.

[SWeko]

If the IEnumerable is indeed a List, Last() will just return list[list.Count-1]

[spender]

Used indiscriminately, this might have disastrous performance characteristics... for instance, called in a loop.

[spender]

Might also lead to false positives if the first/last item is not distinct in the sequence