Cannot implicitly convert type 'sbyte[]' to 'sbyte*'

You need to use the 'fixed' statement to make that conversion:

         sbyte[] StrSbyte = new sbyte[] { 123, 121, 121};
         unsafe
         {
             fixed(sbyte* PtrSbyte = StrSbyte)
             {
                String Strvar = new string(PtrSbyte);
                Console.WriteLine(*PtrSbyte);
                Console.WriteLine(Strvar);
             }
         }