Variable sized arrays in C#

I've found similar threads in the forum but they are all a little different from what I'm trying to do. So, here is the issue:

In Visual Basic, I've tried the code below and it's runs okay.

Public MatDim As Integer ' defined as global variable

Public Class CMatrix
  Public n As Integer = MatDim
  Public a (n, n) As Double
  ...
End Class

Public Sub Main()
  ...
  MatDim = DimFile.ReadLine ()
  Dim a As New CMatrix
  ...
End Sub

In C#, I've tried the same approach (below); didn't work out for me.

public static class GlobalVars {
  public static int MatDim;
}

public class CMatrix {
  public const int n = GlobalVars.MatDim;
  public double [,] a = new double [n, n];
  public double MatDet;
  ...
}

public ActualCodeToRun () {
  ...
  MatDim = 10;
  m = new CMatrix;
  ...
}