Combination of a list - no repeats, specify length

That's actually quite simple. All you need to do is using as many nested for loops as you want numbers in your combination and let the inner loops start at the current outer loop +1. The easiest way would probably be a recursive method if you want to have the combination length variable.

I quickly created this helper class:

 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 
 public class Combinations<T>
 {
     private List<T> m_Items;
     private List<List<T>> m_Result;
     private T[] current;
     private int m_Length;
     private Combinations(List<T> aItems, int aLength)
     {
         m_Items = aItems;
         m_Length = aLength;
         m_Result = new List<List<T>>();
         current = new T[aLength];
     }

     public static List<List<T>> GetCombinations(List<T> aItems, int aLength)
     {
         if (aItems == null || aItems.Count < aLength)
             return new List<List<T>>();
         var context = new Combinations<T>(aItems, aLength);
         
         context.GetCombinations(0, 0);
         return context.m_Result;
     }
     private void GetCombinations(int aStart, int aDepth)
     {
         if (aDepth >= m_Length)
             return;
         int c = m_Items.Count + aDepth - m_Length + 1;
         for (int i = aStart; i < c; i++)
         {
             current[aDepth] = m_Items[i];
             if (aDepth == m_Length-1)
                 m_Result.Add(current.ToList());
             else
                 GetCombinations(i+1, aDepth + 1);
         }
     }
 }

You can use it like this:

 List<int> numbers = new List<int> {2,3,5,7,11,13,17,23};
 List<List<int>> combinations = Combinations<T>.GetCombinations(numbers, 2);

here combinations will contain all combinations with 2 elements of the given list. I've actually tested it with the following code. If does actually print all combinations. The two foreach loops are just to convert the lists into a nice printable form.

 private void Start()
 {
     var n = new List<int> { 1,2,3,4,5,6,7,8,9,10};
     var sb = new StringBuilder();
     for(int i = 1; i < 11; i++)
     {
         var c = Combinations<int>.GetCombinations(n, i);

         sb.Length = 0;
         foreach(var k in c)
         {
             sb.Append('[');
             foreach(int v in k)
             {
                 sb.Append(v).Append(", ");
             }
             sb.Remove(sb.Length-2, 2);
             sb.Append("], ");
         }
         sb.Remove(sb.Length - 2, 2);

         Debug.Log("Combinations of length "+ i + ": " + c.Count + "\n"+ sb.ToString());
     }
 }