Benchmark string concat different approaches

gistfile1.txt

using System;
using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

    public class StringBuilderBenchmark
    {
        [Params(2, 10, 50, 100)] public int ConcatSize { get; set; }

        [Benchmark]
        public string ConcatViaStringBuilder()
        {
            var sb = new StringBuilder();
            for (var i = 0; i < ConcatSize; i++)
            {
                sb.Append(i);
            }

            return sb.ToString();
        }

        [Benchmark]
        public string ConcatViaConcatBoxed()
        {
            var output = string.Empty;
            for (var i = 0; i < ConcatSize; i++)
            {
                output += output + i;
            }

            return output;
        }
        
        [Benchmark]
        public string ConcatViaConcatUnboxed()
        {
            var output = string.Empty;
            for (var i = 0; i < ConcatSize; i++)
            {
                output += output + i.ToString();
            }

            return output;
        }
        
        [Benchmark]
        public string ConcatViaStringFormatBoxed()
        {
            var output = string.Empty;
            for (var i = 0; i < ConcatSize; i++)
            {
                output = $"{output}{i}";
            }

            return output;
        }
        
        [Benchmark]
        public string ConcatViaStringFormatUnboxed()
        {
            var output = string.Empty;
            for (var i = 0; i < ConcatSize; i++)
            {
                output = $"{output}{i.ToString()}";
            }

            return output;
        }
    }
}