Bench.cs

using System;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace Bench
{
    class Program
    {
        static void Main(string[] args)
        {
            BenchmarkRunner.Run<Bench>();
        }
    }

    public class Bench
    {
        private const int iterations = 10000000;
        private List<int> A = new List<int>(iterations);
        private List<int> B = new List<int>(iterations);

        public Bench()
        {
            var rand = new Random();
            for (int i = 0; i < iterations; i++)
            {
                A.Add(rand.Next(10000));
                B.Add(rand.Next(10000));
            }
        }

        [Benchmark]
        public IList<int> RunInterface() => MultVectorsI(A, B);

        [Benchmark]
        public List<int> RunNoInterface() => MultVectors(A, B);

        public List<int> MultVectors(List<int> a, List<int> b)
        {
            int length = a.Count;
            var c = new List<int>(length);
            for (int i = 0; i < length; i++)
            {
                double valA = a[i];
                double valB = b[i];
                c.Add((int)Math.Sqrt(valA * valA + valB * valB));
            }

            return c;
        }

        public IList<int> MultVectorsI(IList<int> a, IList<int> b)
        {
            int length = a.Count;
            var c = new List<int>(length);
            for (int i = 0; i < length; i++)
            {
                double valA = a[i];
                double valB = b[i];
                c.Add((int)Math.Sqrt(valA * valA + valB * valB));
            }

            return c;
        }
    }
}