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

What do you want to order by, other than the known top item? If you don't care, you can do this:

var query = allCountries.OrderBy(x => x.id != 592).ToList();

Basically, "false" comes before "true"...

Admittedly I don't know what this does in LINQ to SQL etc. You may need to stop it from doing the ordering in the database:

var query = allCountries.AsEnumerable()
                        .OrderBy(x => x.id != 592)
                        .ToList();

Answer 2

LINQ is strong in querying collections, creating projections over existing queries or generating new queries based on existing collections. It is not meant as a tool to re-order existing collections inline. For that type of operation it's best to use the type at hande.

Assuming you have a type with a similar definition as below

class Item {
  public int Id { get; set; }
  ..
}

Then try the following

List<Item> list = GetTheList();
var index = list.FindIndex(x => x.Id == 12);
var item = list[index];
list[index] = list[0];
list[0] = item;

Answer 3

public static IEnumerable<T> ServeFirst<T>(this IEnumerable<T> source, 
    Predicate<T> p)
{
    var list = new List<T>();

    foreach (var s in source)
    {
        if (p(s))
            yield return s;
        else
            list.Add(s);
    }

    foreach (var s in list)
        yield return s;
}

Answer 4

Its interesting the number of approaches you find when trying to solve a problem.

var service = AutogateProcessorService.GetInstance();
var allConfigs = service.GetAll();
allConfigs = allConfigs.OrderBy(c => c.ThreadDescription).ToList();
var systemQueue = allConfigs.First(c => c.AcquirerId == 0);
allConfigs.Remove(systemQueue);
allConfigs.Insert(0, systemQueue);

Answer 5

Here is an extension method you might want to use. It moves the element(s) that match the given predicate to the top, preserving order.

public static IEnumerable<T> MoveToTop(IEnumerable<T> list, Func<T, bool> func) {
    return list.Where(func)
               .Concat(list.Where(item => !func(item)));
}

In terms of complexity, I think it would make two passes on the collection, making it O(n), like the Insert/Remove version, but better than Jon Skeet's OrderBy suggestion.