MergeSort with C#

Merge.cs

public class Merge {
  public static int[] data;

  public static void sort(int[] a, int lb, int ub) {
    if (lb < ub) {
      int mid = (lb + ub) / 2;
      Merge.sort(a, lb, mid);
      Merge.sort(a, mid + 1, ub);
      Merge.concat(a, lb, mid, ub);
    }
  }

  private static void concat(int[] a, int lb, int mid, int ub) {
    int i = lb;
    int j = mid + 1;
    int k = lb;

    int[] b = new int[a.Length];

    while (i <= mid && j <= ub) {
      if (a[i] <= a[j]) {
        b[k] = a[i++];
      } else {
        b[k] = a[j++];
      }
      k++;
    }

    if (i > mid) {
      while (j <= ub) {
        b[k++] = a[j++];
      }
    } else {
      while (i <= mid) {
        b[k++] = a[i++];
      }
    }

    for (k = lb; k <= ub; k++) {
      a[k] = b[k];
    }

    Merge.data = new int[a.Length];
    Merge.data = a;
  }

  public static void display(int[] a) {
    Console.WriteLine(Merge.join(", ", a, "[", "]"));
    Console.WriteLine($"Tamanho: {a.Length}");
    Console.WriteLine();
  }

  private static string join(string separator, int[] pieces, string start = "", string end = "") {
    string text = start;

    for (int k = 0; k < pieces.Length; k++) {
      string piece = pieces[k].ToString();
      if (k == pieces.Length - 1) {
        separator = "";
      }
      text += $"{piece}{separator}";
    }

    text += end;

    return text;
  }
}

Program.cs

public class Program
{
  public static void Main(string[] args)
  {
    int[] a = new int[9]{15, 5, 24, 8, 1, 3, 16, 10, 20};

    Merge.display(a); // output: [15, 5, 24, 8, 1, 3, 16, 10, 20]
    Merge.sort(a, 0, a.Length - 1);
    Merge.display(a); // output: [1, 3, 5, 8, 10, 15, 16, 20, 24]
  }
}

Reference:
https://www.youtube.com/watch?v=jlHkDBEumP0