How can I add an item to a list and return a new list

Question

I was asked this question today:

How can I add an item to a list and return that list back?

The code for List<T>.Add(T) returns void. So you can't do something like this:

var list = new List<string>{"item1","item2"}; var newList = list.Add("item3"); 

This is related to using AutoMapper, although that part isn't particularly important.

Answer 1

One option is Linq, with Concat:

var list = new List<string>{"item1", "item2"}; var newList = list.Concat(new[] { "item3" }).ToList(); 

In typical Linq fashion, list stays the same, and newList contains all the items from list as well as the items in the new list, in this case just "item3".

You can skip the .ToList() to keep the IEnumerable<string> result if that fits your use case.

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If you find yourself doing this often with individual items, you can use something like this extension method to pass them without the new[] { ... } syntax:

public static IEnumerable<T> ConcatItems<T>(this IEnumerable<T> source, params T[] items) {     return source.Concat(items); } 

Because of the params array the earlier example becomes:

var list = new List<string>{"item1", "item2"}; var newList = list.ConcatItems("item3").ToList(); 

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Make sure not to mix this up with Union, which removes duplicate items. (Searching for those duplicates is overhead that you probably don't want!)