How to filter a list?

Question

This is a method that should take already assigned users out of a list and keep non-assigned ones in a list. The GuidList has the userId added to it on a button click. The profileList is used to populate a gridView.

Here is the code:

private VList<VW_profiles> FilterAssigned(VList<VW_profiles> profileList)
{
 VList<VW_profiles> sortedList = new VList<VW_profiles>();
 foreach(VW_profiles profile in profileList)
 {
  if(GuidList.Count > 0)
  {
   foreach(Guid userId in GuidList)
   {
    if(profile.UserId != userId)
    {
     sortedList.Add(profile)
    }
   }
  }  
  else
  {
   sortedList = profileList;
  }
 }
 return sortedList;
}

Now here's my problem. Everythings seems to work well up until all of the items in the profileList have also been added to the GuidList. Then instead of doing a negate on the two Guid ID's, we start adding everyone in again. Does anyone have any suggestions on how to do this is a more effecient way and to avoid the adding in once we've taken everything out.

Thanks!

Answer 1

If VList<T> is a List<T>, then you can do this:

profileList.RemoveAll(profile => GuidList.Contains(profile.UserId));

If performance is an issue and there are LOTS of Guids to remove, then you can make GuidList a HashSet<Guid>.

Edit Based on comments: If you don't want to modify the original list, then do this:

var filtered = new VList<VW_profiles>(
    profileList.Where(profile => !GuidList.Contains(profile.UserId)));

Edit If you are not using a List<T>, here is a method you can use on resizable lists implementing IList<T> and one you can use on arrays (T[]). By only ever removing items from the end of the list, what would be an O(n²) algorithm will be O(n) for most implementations of IList<T>.

public static void RemoveAll<T>(this IList<T> list, Predicate<T> match)
{
    if (list == null)
        throw new ArgumentNullException("list");
    if (match == null)
        throw new ArgumentNullException("match");
    if (list is T[])
        throw new ArgumentException("Arrays cannot be resized.");

    // early out
    if (list.Count == 0)
        return;

    // List<T> provides special handling
    List<T> genericList = list as List<T>;
    if (genericList != null)
    {
        genericList.RemoveAll(match);
        return;
    }

    int targetIndex = 0;
    for (int i = 0; i < list.Count; i++)
    {
        if (!match(list[i]) && targetIndex != i)
        {
            list[targetIndex] = list[i];
            targetIndex++;
        }
    }

    // Unfortunately IList<T> doesn't have RemoveRange either
    for (int i = list.Count - 1; i >= targetIndex; i--)
    {
        list.RemoveAt(i);
    }
}

public static void RemoveAll<T>(ref T[] array, Predicate<T> match)
{
    if (array == null)
        throw new ArgumentNullException("array");
    if (match == null)
        throw new ArgumentNullException("match");

    int targetIndex = 0;
    for (int i = 0; i < array.Length; i++)
    {
        if (!match(array[i]) && targetIndex != i)
        {
            array[targetIndex] = array[i];
            targetIndex++;
        }
    }

    if (targetIndex != array.Length)
    {
        Array.Resize(ref array, targetIndex);
    }
}

Answer 2

Your problem is in this code:

foreach(Guid userId in GuidList)
{
    if(profile.UserId != userId)
    {
        sortedList.Add(profile)
    }
}

It should be more like:

bool inList = false;
foreach(Guid userId in GuidList)
{
    if(profile.UserId == userId)
    {
        inList = true;
    }
}
if (!inList)
    sortedList.Add(profile)

or, the more LINQ-style:

bool inList = GuidList.Any(x => x == profile.UserId);
if (!inList)
    sortedList.Add(profile)

your current code is more like:

GuidList.Where(x => x != profile.UserId)
        .Foreach(x => sortedList.Add(x));

which I'm thinking isn't what you want :)