Use LINQ to move item to top of list

Question

Is there a way to move an item of say id=10 as the first item in a list using LINQ?

Item A - id =5
Item B - id = 10
Item C - id =12
Item D - id =1

In th

Answer 1

To also check if the item was found without Exception, something like:

var allCountries = repository.GetCountries();
var lookup = allCountries.ToLookup(x => x.id == 592);  
var finalList = lookup[true].Concat(lookup[false]).ToList();
if ( lookup[true].Count() != 1 ) YouAreInTrouble();

Answer 2

I wrote a static extension method to do this. Note this doesn't preserve the order, it simply swaps the item out. If you needed to preserve the order you should do a rotate not a simple swap.

/// <summary>
/// Moves the item to the front of the list if it exists, if it does not it returns false
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="collection"></param>
/// <param name="predicate"></param>
/// <returns></returns>
public static bool MoveToFrontOfListWhere<T>(this List<T> collection, Func<T, bool> predicate)
{
    if (collection == null || collection.Count <= 0) return false;

    int index = -1;
    for (int i = 0; i < collection.Count; i++)
    {
        T element = collection.ElementAt(i);
        if (!predicate(element)) continue;
        index = i;
        break;
    }

    if (index == -1) return false;

    T item = collection[index];
    collection[index] = collection[0];
    collection[0] = item;
    return true;
}

Answer 3

var allCountries = repository.GetCountries();
allCountries.OrderByDescending(o => o.id == 12).ThenBy(o => o.id) 

This will insert the object with id=12 at the top of the list and rotate the rest down, preserving the order.

Answer 4

Linq generallyworks on Enumerables, so it doesn't now that the underlying type is a collection. So for moving the item on top of the list I would suggest using something like (if you need to preserve the order)

var idx = myList.FindIndex(x => x.id == 592);
var item = myList[idx];
myList.RemoveAt(idx);
myList.Insert(0, item);

If your function returns only an IEnumerable, you can use the ToList() method to convert it to a List first

If you don't preserve the order you can simply swap the values at position 0 and position idx

Answer 5

I know this a old question but I did it like this

class Program
{
    static void Main(string[] args)
    {
        var numbers = new int[] { 5, 10, 12, 1 };

        var ordered = numbers.OrderBy(num => num != 10 ? num : -1);

        foreach (var num in ordered)
        {
            Console.WriteLine("number is {0}", num);
        }

        Console.ReadLine();
    }
}

this prints:

number is 10
number is 1
number is 5
number is 12

Answer 6

You can "group by" in two groups with Boolean key, and then sort them

var finalList= allCountries
                .GroupBy(x => x.id != 592)
                .OrderBy(g => g.Key)
                .SelectMany(g => g.OrderBy(x=> x.id ));