IsContainedIn

Returns true if the elements of the first collection are contained in the second one regardless of order, false otherwise.

• Use IEnumerable.GroupBy() to create groups for each distinct value in each collection, IEnumerable.ToDictionary() to convert the result to a Dictionary.
• Use IEnumerable.Distinct() to find the distinct values from the first collection and loop over them using a foreach loop.
• Use Dictionary.ContainsKey() to check that each distinct value exists in the second collection and compare the count for each one.
• Return false if any value is not found in the second collection or if any count in it is lower than in the first one, true otherwise.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static bool IsContainedIn<T>(IEnumerable<T> a, IEnumerable<T> b) 
  {
    Dictionary<T,int> dA = a.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
    Dictionary<T,int> dB = b.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
    foreach(T val in a.Distinct()) {
      if (!dB.ContainsKey(val)) return false;
      if (dA[val] > dB[val]) return false;
    }
    return true;
  }
}

int[] a = {1, 4};
int[] b = {2, 4, 1};

_30s.IsContainedIn(a, b); // true