String to upper

StringToUpper.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace StringToUpper
{
	class ParallelString
	{
		string str;
		ParallelString(string str){
			this.str = str;
		}
		public void ParallelChangeCase() {
			string []sub_str = str.Substring ();
			Parallel.ForEach (0, str.Length, i => {

			});
		}
	}
	class Matrix
	{
		long [,]matA;
		public void RandomizeMat (int n, int m){
			matA = new long[n, m];
			var rnd = new Random (int.Parse(DateTime.Now.ToString("HHmmssfff"))+(int)(n));
			for (long i = 0 ; i < n ; ++i) {
				for (long j = 0; j < m; ++j) {
					matA [i,j] = rnd.Next (0, n);
				}
			}
		}
		private int IsPrime(long n) {
			int res = 1;
			Parallel.For (2,(int)Math.Ceiling(Math.Sqrt(n))+1, i => {
				if(n%i == 0){
					res = 0;
				}
			});
			if (n == 2) {
				res = 1;
			} else if (n <= 1) {
				res = -1;
			}
			return res;
		}
		private bool IsOdd(long n) {
			return ((n & 1) == 1);
		}
		public void ParallelPrimeReplace(){
			Parallel.For (0, matA.GetLength (0), i => {
				Parallel.For(0, matA.GetLength(1), j => {
					matA[i,j] = IsPrime(matA[i,j]);
				});
			});
		}
		public void ParallelOddSqEvenCuReplace(){
			Parallel.For (0, matA.GetLength (0), i => {
				Parallel.For(0, matA.GetLength(1), j => {
					matA[i,j] = (long) (IsOdd(Thread.CurrentThread.ManagedThreadId)?Math.Pow(matA[i,j],2):Math.Pow(matA[i,j],3));
				});
			});
		}
		public override string ToString() {
			string res = "";
			for (long i = 0; i < matA.GetLength (0); ++i) {
				for (long j = 0; j < matA.GetLength (1); ++j) {
					res += (matA [i, j] + "\t");
				}
				res += "\n";
			}
			return res;
		}
	}
	class MainClass
	{

		public static void Main (string[] args)
		{
			var mat = new Matrix ();
			mat.RandomizeMat (5,5);
			Console.WriteLine (mat);
			mat.ParallelPrimeReplace ();
			Console.WriteLine (mat);
			mat.RandomizeMat (5,5);
			Console.WriteLine (mat);
			mat.ParallelOddSqEvenCuReplace ();
			Console.WriteLine (mat);
		}
	}
}