String building comparison

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace StringConcat
{
  class Program
  {
    static void Main(string[] args)
    {
      var prng = new Random();

      for (int n = 0; n < ArraySize; n++)
      {
          var randomCharacters = "";
          for (int i = 0; i < StringSize; i++)
          {
            randomCharacters += Char.ConvertFromUtf32(prng.Next(65, 90));
          }
          StringArray[n] = randomCharacters;
      }
      Console.WriteLine("PHP version (in C#)         = {0:F4}s", Measure(PhpComparison));
      Console.WriteLine("Using prebuilt array        = {0:F4}s", Measure(UsePregeneratedStrings));
      Console.WriteLine("Using StringBuilder, random = {0:F4}s", Measure(UseBuilder));
      Console.WriteLine("Using StringBuilder, array  = {0:F4}s", Measure(UseBuilderPregenerated));

      Console.ReadLine();
    }
    
    private static int ArraySize = 1000000;
    private static int StringSize = 20;
    private static String[] StringArray = new String[1000000];

    private static Func<Action,Double> Measure = (f) =>
      {
        var watch = new Stopwatch();
        var frequency = Stopwatch.Frequency;
        var nanosecondsPerTick = frequency / 1.0e9;

        var minTime = 
          Enumerable
            .Range(1, 5)
            .Select(n =>
              {
                watch.Start();
                f();
                var ticks = watch.Elapsed.TotalSeconds;
                watch.Reset();
                return ticks;
              })
            .Min();

        return minTime;
      };

    private static Action PhpComparison = () =>
      {
        var prng = new Random();

        var stringBuffer = new String[ArraySize];

        Func<String> fillRandomChars = () =>
        {
          var randomCharacters = "";
          for (int i = 0; i < StringSize; i++)
          {
            randomCharacters += Char.ConvertFromUtf32(prng.Next(65, 90));
          }
          return randomCharacters;
        };

        for (int i = 0; i < ArraySize; i++)
        {
          stringBuffer[i] = fillRandomChars();
        }

        String.Concat(stringBuffer);
      };

    private static Action UsePregeneratedStrings = () =>
      {
        var stringBuffer = new String[ArraySize];

        for (int i = 0; i < ArraySize; i++)
        {
          stringBuffer[i] = StringArray[i];
        }

        String.Concat(stringBuffer);
      };

    private static Action UseBuilder = () =>
    {
      var prng = new Random();

      var builder = new StringBuilder();

      Func<String> fillRandomChars = () =>
      {
        var randomCharacters = "";
        for (int i = 0; i < StringSize; i++)
        {
          randomCharacters += Char.ConvertFromUtf32(prng.Next(65, 90));
        }
        return randomCharacters;
      };

      for (int i = 0; i < ArraySize; i++)
      {
        builder.Append(fillRandomChars());
      }

      builder.ToString();
    };

    private static Action UseBuilderPregenerated = () =>
    {
      var builder = new StringBuilder();

      for (int i = 0; i < ArraySize; i++)
      {
        builder.Append(StringArray[i]);
      }

      builder.ToString();
    };
  }
}