Spot the Bug: Unmanaged Memory Traps

sometimes i forgot how good life is in the managed lane.  then i do some unmanaged work and get a good reality check.

let us look at the following structures:

    [structlayout(layoutkind.explicit, pack = 1)]
    public struct pageheader
    {
        [fieldoffset(0)] public long marker;

        [fieldoffset(8)] public ushort lower;

        [fieldoffset(10)] public ushort upper;

        [fieldoffset(12)] public int overflowsize;

        [fieldoffset(16)] public int itemcount;
    }


    [structlayout(layoutkind.explicit, pack = 1)]
    public struct fileheader
    {
        [fieldoffset(0)] public long marker;

        [fieldoffset(8)] public logheader active;

        [fieldoffset(44)] public logheader backup;

        [fieldoffset(80)] public treerootheader root;

        [fieldoffset(142)] public treerootheader freespace;
    }

    [structlayout(layoutkind.explicit, pack = 1)]
    public struct treerootheader
    {
        [fieldoffset(0)] public long rootpagenumber;
        [fieldoffset(8)] public long branchpages;
        [fieldoffset(16)] public long leafpages;
        [fieldoffset(32)] public long overflowpages;
        [fieldoffset(40)] public long pagecount;
        [fieldoffset(48)] public long entriescount;
        [fieldoffset(56)] public int depth;
        [fieldoffset(60)] public treeflags flags;
    }

    [structlayout(layoutkind.explicit, pack = 1)]
    public struct logheader
    {
        [fieldoffset(0)] public long marker;

        [fieldoffset(8)] public long lastlog;

        [fieldoffset(16)] public long lastlogpage;

        [fieldoffset(24)] public int itemcount;

        [fieldoffset(28)] public long options;
    }

and now we have the following code:

private static unsafe void main()
  {
      intptr pageptr = marshal.allochglobal(4096);
   
      var pageheader = (pageheader*) pageptr.topointer();
      pageheader->itemcount = 2;
      pageheader->marker = 0x128314543423;
      pageheader->overflowsize = 32;
  
      fileheader* fileheader = (fileheader*) pageheader + sizeof (pageheader);
   
      fileheader->root.branchpages = 0;
   
   
      marshal.freehglobal(pageptr);
  }

the fun part about this code is that it would silently corrupt the state of the process.

here is what happens when you run it under the debugger:

can you figure out why?