How to convert a Byte Array to decimal value and vice versa in c# 4.0 framework

Question

How to convert a Byte Array to decimal value and vice versa in c# 4.0 framework.
The following code working in 2.0 and 3.5 , but giving exception in 4.0:--

byte[] bytearray = new bytearray[16];

MemoryStream m2 = new MemoryStream(bytearray);

         BinaryReader reader = new BinaryReader(m2);

    decimal dec = reader.ReadDecimal();

        Console.WriteLine(dec.ToString());

           Console.ReadLine();

          m2.Close();

      reader.Close();