DllImportAttribute , SetLastError is ignored

[NN---]

using System;
using System.Console;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Reflection;

namespace test2
{
    module Program
    {
      [DllImport("a.dll", SetLastError=true)]
      public extern F() : void;

      Main() : void
      {          
        def t = Type.GetType("test2.Program");
        def f = t.GetMethod("F");

        foreach (at in f.GetCustomAttributes(false))
        {
          | dat is DllImportAttribute => WriteLine(dat.SetLastError);
          | _ => ()
        }
      }
   }
}

Output is false , not true.