Does .NET have a way to check if List a contains all items in List b?

Question

I have the following method:

namespace ListHelper
{
    public class ListHelper<T>
    {
        public static bool ContainsAllItems(List<T> a, List<T> b)
        {
            return b.TrueForAll(delegate(T t)
            {
                return a.Contains(t);
            });
        }
    }
}

The purpose of which is to determine if a List contains all the elements of another list. It would appear to me that something like this would be built into .NET already, is that the case and am I duplicating functionality?

Edit: My apologies for not stating up front that I'm using this code on Mono version 2.4.2.

Carpo

See also #333473 – Broccoli

Your algorithm is quadratic O(nm). If the lists are sorted, testing if one is a subset of another should be possible in O(n+m) time. – Broccoli

Answer 1

If you're using .NET 3.5, it's easy:

public class ListHelper<T>
{
    public static bool ContainsAllItems(List<T> a, List<T> b)
    {
        return !b.Except(a).Any();
    }
}

This checks whether there are any elements in b which aren't in a - and then inverts the result.

Note that it would be slightly more conventional to make the method generic rather than the class, and there's no reason to require List<T> instead of IEnumerable<T> - so this would probably be preferred:

public static class LinqExtras // Or whatever
{
    public static bool ContainsAllItems<T>(this IEnumerable<T> a, IEnumerable<T> b)
    {
        return !b.Except(a).Any();
    }
}

Carlyncarlynn

This is untested, but wouldn't return b.Except(a).Empty(); be much more readable ? – Salome

Except that Empty() doesn't return a boolean. It returns an IEnumerable<T> with no items. – Chapman

This requires Linq right? If so, I don't think that's available in Mono, which is what I'm using at the moment. – Carpo

You can use LINQ to Objects in Mono, I believe... but it would be helpful if you'd state the requirements in the question to start with. Which version of Mono are you using? – Carlyncarlynn

I've just checked the sources for Mono 2.4.2.3, and it definitely includes LINQ to Objects. – Carlyncarlynn

If the lists are length n and m, what's the time complexity of this algorithm? – Broccoli

@ColonelPanic: Assuming no hash collisions, O(n+m). – Carlyncarlynn

If two lists are identical, this will return false, because there wouldn't be Any() elements found after the Except() – Exit

@Bojan: No, it will return true, because the expression starts with !. See dotnetfiddle.net/QdXPy7 for a runnable example. – Carlyncarlynn

Answer 2

Included in .NET 4: Enumerable.All

public static bool ContainsAll<T>(IEnumerable<T> source, IEnumerable<T> values)
{
    return values.All(value => source.Contains(value));
}

Morton

Enumerable.All existed before .Net 4.0, and here it's a O(N^2) operation as opposed to the one in the accepted answer. – Parttime

Answer 3

Just for fun, @JonSkeet's answer as an extension method:

/// <summary>
/// Does a list contain all values of another list?
/// </summary>
/// <remarks>Needs .NET 3.5 or greater.  Source:  https://mcmap.net/q/186357/-does-net-have-a-way-to-check-if-list-a-contains-all-items-in-list-b </remarks>
/// <typeparam name="T">list value type</typeparam>
/// <param name="containingList">the larger list we're checking in</param>
/// <param name="lookupList">the list to look for in the containing list</param>
/// <returns>true if it has everything</returns>
public static bool ContainsAll<T>(this IEnumerable<T> containingList, IEnumerable<T> lookupList) {
    return ! lookupList.Except(containingList).Any();
}

Drumstick

similarly: Contains Any = public static bool ContainsAny<T>(this IEnumerable<T> haystack, IEnumerable<T> needle) { return haystack.Intersect(needle).Count() > 0; }. I tried some quick performance comparisons to haystack.Count() - 1 >= haystack.Except(needle).Count(); and Intersect seemed to do better most of the time. – Drumstick

sheesh...use Any() not Count() > 0: public static bool ContainsAny<T>(this IEnumerable<T> haystack, IEnumerable<T> needle) { return haystack.Intersect(needle).Any(); } – Drumstick

Answer 4

I know a way using LinQ methods. It's a bit weird to read, but works pretty well

var motherList = new List<string> { "Hello", "World", "User };
var sonList = new List<string> { "Hello", "User" };

You want to check if sonList is totally in motherList

To do so:

sonList.All(str => moterList.Any(word => word == str));

// Reading literally, would be like "For each of all items 
// in sonList, test if it's in motherList

Please check it on and see if works there too. Hope it helps ;-)

Objectionable

This works. But please correct the typo in this line: bool result = sonList.All(str => motherList.Any(word => word == str)); – Tyndareus

Answer 5

You could also use another way. Override equals and use this

public bool ContainsAll(List<T> a,List<T> check)
{
   list l = new List<T>(check);
   foreach(T _t in a)
   {
      if(check.Contains(t))
      {
         check.Remove(t);
         if(check.Count == 0)
         {
            return true;
         }
      }
      return false;
   }
}

Ekaterinodar

list l = new List<T>(check); I dont think this would compile and if it does, its totally unecessary as check is already a list – Presentationism

Answer 6

    public static bool ContainsAll<T>(this IEnumerable<T> containingList, IEnumerable<T> lookupList)
    {
        return containingList.Intersect(lookupList).Count() == lookupList.Count();
    }

Pliocene

Thank you for your interest in contributing to the Stack Overflow community. This question already has quite a few answers—including one that has been extensively validated by the community. Are you certain your approach hasn’t been given previously? If so, it would be useful to explain how your approach is different, under what circumstances your approach might be preferred, and/or why you think the previous answers aren’t sufficient. Can you kindly edit your answer to offer an explanation? – Beersheba