Removing elements from binding list

You could try:

UserList = UserList.Where(x => x.id == ID).ToList(); 

If you use RemoveAll() inside a generic class that you intend to be used to hold a collection of any type object, like this:

public class SomeClass<T>
{

    internal List<T> InternalList;

    public SomeClass() { InternalList = new List<T>(); }

    public void RemoveAll(T theValue)
    {
        // this will work
        InternalList.RemoveAll(x =< x.Equals(theValue));
        // the usual form of Lambda Predicate 
        //for RemoveAll will not compile
        // error: Cannot apply operator '==' to operands of Type 'T' and 'T'
        // InternalList.RemoveAll(x =&amp;gt; x == theValue);
    }
}

This content is taken from here.

It's not working because you are working on a copy of the list which you created by calling ToList().

BindingList<T> doesn't support RemoveAll(): it's a List<T> feature only, so:

IReadOnlyList<User> usersToRemove = UserList.Where(x => (x.id == ID)).
                                             ToList();

foreach (User user in usersToRemove)
{
    UserList.Remove(user);
}

We're calling ToList() here because otherwise we'll enumerate a collection while modifying it.