Using LINQ to remove elements from a List

Question

Say that I have LINQ query such as:

var authors = from x in authorsList
              where x.firstname == \"Bob\"
              select x;

Answer 1

Well, it would be easier to exclude them in the first place:

authorsList = authorsList.Where(x => x.FirstName != "Bob").ToList();

However, that would just change the value of authorsList instead of removing the authors from the previous collection. Alternatively, you can use RemoveAll:

authorsList.RemoveAll(x => x.FirstName == "Bob");

If you really need to do it based on another collection, I'd use a HashSet, RemoveAll and Contains:

var setToRemove = new HashSet<Author>(authors);
authorsList.RemoveAll(x => setToRemove.Contains(x));

Answer 2

You cannot do this with standard LINQ operators because LINQ provides query, not update support.

But you can generate a new list and replace the old one.

var authorsList = GetAuthorList();

authorsList = authorsList.Where(a => a.FirstName != "Bob").ToList();

Or you could remove all items in authors in a second pass.

var authorsList = GetAuthorList();

var authors = authorsList.Where(a => a.FirstName == "Bob").ToList();

foreach (var author in authors)
{
    authorList.Remove(author);
}

Answer 3

If you really need to remove items then what about Except()?
You can remove based on a new list, or remove on-the-fly by nesting the Linq.

var authorsList = new List<Author>()
{
    new Author{ Firstname = "Bob", Lastname = "Smith" },
    new Author{ Firstname = "Fred", Lastname = "Jones" },
    new Author{ Firstname = "Brian", Lastname = "Brains" },
    new Author{ Firstname = "Billy", Lastname = "TheKid" }
};

var authors = authorsList.Where(a => a.Firstname == "Bob");
authorsList = authorsList.Except(authors).ToList();
authorsList = authorsList.Except(authorsList.Where(a=>a.Firstname=="Billy")).ToList();

Answer 4

This is a very old question, but I found a really simple way to do this:

authorsList = authorsList.Except(authors).ToList();

Note that since the return variable authorsList is a List<T>, the IEnumerable<T> returned by Except() must be converted to a List<T>.

Answer 5

You can remove in two ways

var output = from x in authorsList
             where x.firstname != "Bob"
             select x;

or

var authors = from x in authorsList
              where x.firstname == "Bob"
              select x;

var output = from x in authorsList
             where !authors.Contains(x) 
             select x;

I had same issue, if you want simple output based on your where condition , then first solution is better.

Answer 6

i think you just have to assign the items from Author list to a new list to take that effect.

//assume oldAuthor is the old list
Author newAuthorList = (select x from oldAuthor where x.firstname!="Bob" select x).ToList();
oldAuthor = newAuthorList;
newAuthorList = null;