Why? Some sort of security as it seems but where to adjust?

Reproducing:

using System;
using System.IO;

class test
{
  [STAThread]
  static void Main()
  {
     using (StreamWriter wr = new StreamWriter(@"c:\temp\test.txt"))
      {
         sw.WriteLine("Hello from C#");
      }
  }
}

Compile with csc and try to ShellExecute from say VFP. Fails with error code 5.
PS: Executing account is member of Administrators. Tried ngen install too.
TIA
Cetin