List(T) RemoveAll() not working as intended...?

Let's say I have a list of User objects with two properties...ID and Name

List<User> lst = List<User>();

I fill it up with a bunch of Users. Ok, now I want to trim 开发者_StackOverflow中文版my list using RemoveAll() and this function.

private Boolean IsExisting(int id) {
//blah blah
return true;
//blah blah
return false;
}

So I use this statement:

gdvFoo.DataSource = lst.RemoveAll(t => IsExisting(t.ID));

It is my understanding that whenever IsExisting returns true that element should be removed from lst, but what happens, strangely enough, is it returns an integer?, not a truncated list and I received the following error message:

Data source is an invalid type. It must be either an IListSource, IEnumerable, or IDataSource.>

List.RemoveAll method

The method removes all matching instances from the list on which you called it. This modifies the existing list, rather than returning a new one.

The return value is the number of rows removed.

RemoveAll() returns the number of elements removed. You need to do this:

lst.RemoveAll(t => IsExisting(t.ID)); 
gdvFoo.DataSource = lst;

The docs are very clear about what's going on:

Return Value Type: System.Int32 The number of elements removed from the List .

Perhaps the following Linq would be more in line with your expectations?

lst.Except(t => IsExisting(t.ID)).ToList();

Instead of RemoveAll(), you could try using IEnumerable's filter where you would say something like :

var filteredList = lst.Where(item => IsExisting(item.Id)) 

This makes the code a little more easier to read and focusses on the objective of the task at hand, rather than how to look at implementing it.

List<T>.RemoveAll(...) has a return type of int which is not an IListSource, IEnumerable nor IDataSource

The RemoveAll is modifying the list and returning the number of items removed. You just set your datasource to the list in a second step.

lst.RemoveAll(t => IsExisting(t.ID));
gdvFoo.DataSource = lst;