Question

If there any short way besides foreach loop and new collection creation to add object after specific object in existing List ?

Just an example:

“amy”,”jerry”,”tony”,”amy”,”jack”.
I want to add “simon” after each “amy” in short way

Answer 1

You can use Linq to do this.

foreach (var item in values
          .Select((o, i) => new { Value = o, Index = i })
          .Where(p => p.Value == "amy")
          .OrderByDescending(p => p.Index))
{
    if (item.Index + 1 == values.Count) values.Add("simon");
    else values.Insert(item.Index + 1, "simon");
} 

Uses a foreach but you can put it into a extension method to keep the code clear.

Extension method

You can easily put this into a extension method.

public static void AddAfterEach<T>(this List<T> list, Func<T, Boolean> condition, T objectToAdd) 
{
    foreach (var item in list.Select((o, i) => new { Value = o, Index = i }).Where(p => condition(p.Value)).OrderByDescending(p => p.Index))
    {
        if (item.Index + 1 == list.Count) list.Add(objectToAdd);
        else list.Insert(item.Index + 1, objectToAdd);
    } 
}

Now the call:

List<String> list = new List<String>() { "amy","jerry","tony","amy","jack" };
list.AddAfterEach(p => p == "amy", "simon");