I'm trying to wrap calls the USPS Address Matching System API.

Am I missing something in my call to the API?

My calls to the API are declared as follows:

   <DllImport("zip4_w32.dll", BestFitMapping:=True,
CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Auto, _
       EntryPoint:="z4openSTD", ExactSpelling:=True, PreserveSig:=True,
SetLastError:=True, ThrowOnUnmappableChar:=True)> _
   Shared Function Open() As Int32
   End Function

   <DllImport("zip4_w32.dll", BestFitMapping:=True,
CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Auto, _
       EntryPoint:="z4closeSTD", ExactSpelling:=True, PreserveSig:=True,
SetLastError:=True, ThrowOnUnmappableChar:=True)> _
   Shared Function Close() As Int32
   End Function

   <DllImport("zip4_w32.dll", BestFitMapping:=True,
CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Auto, _
       EntryPoint:="z4adrinqSTD", ExactSpelling:=True, PreserveSig:=True,
SetLastError:=True, ThrowOnUnmappableChar:=True)> _
   Shared Function Inquire(<MarshalAs(UnmanagedType.Struct)> ByRef
Zip4Struct As structZIP4_PARM) As Int32
   End Function

The zip4.h file defines the structure being passed to the Inquire method as:

/***************************************************************************/
/*   Parameter list for z4adrinq() and z4xrfinq() calls.  Reserved         */
/*   fields are for future use, do not access these fields.  Size of this  */
/*   record cannot be changed.                                             */
/*   NOTE:  Only fields containing +1 in the length are null terminated.   */
/***************************************************************************/
typedef struct
{
                  /*********** input data ***********/

  char     rsvd0[4];               /* reserved space                 */
  char     iadl1[50+1];            /* input delivery address         */
  char     iadl2[50+1];            /* input firm name                */
  char     ictyi[50+1];            /* input city                     */
  char     istai[2+1];             /* input state                    */
  char     izipc[10+1];            /* input ZIP+4 code               */
  char     iprurb[28+1];           /* input urbanization name        */
  char     iadl3[50+1];            /* input second address line      */
  char     rsvd1[98];              /*reserved for future use         */
                  /********** returned data *********/
  char     dadl3[50+1];            /* standardized 2nd delivery addr */
  char     dadl1[50+1];            /* standardized delivery address  */
  char     dadl2[50+1];            /* standardized firm name         */
  char     dlast[50+1];            /* standardized city/state/zip    */
  char     dprurb[28+1];           /* output PR urbanization name    */
  char     dctys[28+1];            /* main post office city          */
  char     dstas[2+1];             /* main post office state         */
  char     dctya[28+1];            /* standardized city              */
  char     abcty[13+1];            /* standardized city abbreviation */
  char     dstaa[2+1];             /* standardized state             */
  char     zipc[5+1];              /* 5-digit zip code               */
  char     addon[4+1];             /* ZIP+4 addon code               */
  char     dpbc[3+1];              /* delivery point bar code        */
  char     cris[4+1];              /* carrier route                  */
  char     county[3+1];            /* FIPS county code               */
  short    respn;                  /* number of returned responses   */
  char     retcc;                  /* return code                    */
  char     adrkey[12];             /* address key (for indexing)     */
  char     auto_zone_ind;          /* A, B, C or D                   */
  char     elot_num[4+1];          /* eLOT Number                    */
  char     elot_code;              /* eLOT Ascending/Descending Flag */
                 /******* parsed input data ********/
  char     ppnum[10+1];            /* Primary Number                 */
  char     psnum[8+1];             /* Secondary Number               */
  char     prote[3+1];             /* Rural Route Number             */
  char     punit[4+1];             /* Secondary Number Unit          */
  char     ppre1[2+1];             /* First or Left Pre-direction    */
  char     ppre2[2+1];             /* Second or Right Pre-direction  */
  char     psuf1[4+1];             /* First or Left Suffix           */
  char     psuf2[4+1];             /* Second or Right Suffix         */
  char     ppst1[2+1];             /* First or Left Post-direction   */
  char     ppst2[2+1];             /* Second or Right Post-direction */
  char     ppnam[28+1];            /* Primary Name                   */

  char     mpnum[10+1];            /* Matched primary number.        */
  char     msnum[8+1];             /* Matched secondary number.      */
  char     pmb[3+1];               /* PMB Unit Designator            */
  char     pmbnum[8+1];            /* PMB Number                     */
  char     mlevl;                  /* Reserved Use                   */
  char     rsvd2[85];              /* reserved for future use        */

  struct {                         /************ footnotes ***********/
     char  a;                      /* zip corrected                  */
     char  b;                      /* city/state corrected           */
     char  c;                      /* invalid city/state/zip         */
     char  d;                      /* no zip assigned                */
     char  e;                      /* ZIP assigned for mult response */
     char  f;                      /* no zip available               */
     char  g;                      /* part of firm moved to address  */
     char  h;                      /* secondary number missing       */
     char  i;                      /* insufficient/incorrect data    */
     char  j;                      /* dual input -- used PO BOX      */
     char  k;                      /* dual input -- used non-PO BOX  */
     char  l;                      /* del addr component add/del/chg */
     char  m;                      /* street name spelling changed   */
     char  n;                      /* delivery addr was standardized */
     char  o;                      /* reserved for future use        */
     char  p;                      /* better delivery addr exists    */
     char  q;                      /* reserved for future use        */
     char  r;                      /* no match due to EWS            */
     char  s;                      /* invalid secondary number       */
     char  t;                      /* multiple caused by magnet rule */
     char  u;                      /* unofficial po name             */
     char  v;                      /* unverifiable city/state        */
     char  w;                      /* small town default             */
     char  x;                      /* unique ZIP code default        */
     char  y;                      /* reserved for future use        */
     char  z;                      /* ZIP move match                 */
     char  rsvd3[6];               /* reserved for future use        */
     } foot;

  ADDR_REC stack[10];              /********** record stack **********/
  char     rsvd4[194];             /* reserved for future use        */
} ZIP4_PARM;

and my struct is defined as:

   Structure structZIP4_PARM
       'Input Data
       Dim rsvd0 As Char()                                 ' Reserved for
Future Use
       <VBFixedString(50)> Dim iadl1 As String             ' Input Delivery
Address
       <VBFixedString(50)> Dim iadl2 As String             ' Input Firm Name
       <VBFixedString(50)> Dim ictyi As String             ' Input City
       <VBFixedString(2)> Dim istai As String              ' Input State
       <VBFixedString(10)> Dim izipc As String             ' Input ZIP+4 Code
       <VBFixedString(28)> Dim iprurb As String            ' Input
Urbanization Name
       <VBFixedString(50)> Dim iadl3 As String             ' Input Second
Address Line
       Dim rsvd1 As Char()                                 ' Reserved for
Future User

       'Output Data
       <VBFixedString(50)> Dim dadl3 As String             ' Standardized
2nd Delivery Address
       <VBFixedString(50)> Dim dadl1 As String             ' Standardized
Delivery Address
       <VBFixedString(50)> Dim dadl2 As String             ' Standardized
Firm Name
       <VBFixedString(50)> Dim dlast As String             ' Standardized
City/State/Zip
       <VBFixedString(28)> Dim dprurb As String            ' Output PR
Urbanization Name
       <VBFixedString(28)> Dim dctys As String             ' Main Post
Office City
       <VBFixedString(2)> Dim dstas As String              ' Main Post
Office State
       <VBFixedString(28)> Dim dctya As String             ' Standardized
City
       <VBFixedString(13)> Dim abcty As String             ' Standardized
City Abbreviation
       <VBFixedString(2)> Dim dstaa As String              ' Standardized
State
       <VBFixedString(5)> Dim zipc As String               ' 5-digit zip Code
       <VBFixedString(4)> Dim addon As String              ' ZIP+4 addon Code
       <VBFixedString(3)> Dim dpbc As String               ' Delivery Point
Bar Code
       <VBFixedString(4)> Dim cris As String               ' Carrier Route
       <VBFixedString(3)> Dim county As String             ' FIPS County Code
       Dim respn As Int16                                  ' Number of
Returned Responses
       Dim retcc As Char                                   ' Return Code
       Dim adrkey As Char()                                ' Address Key
(for Indexing)
       Dim auto_zone_ind As Char                           ' A, B, C, or D
       <VBFixedString(4)> Dim elot_num As String           ' eLOT Number
       Dim elot_code As Char                               ' eLOT
Ascending/Descending Flag

       ' Parsed Input Data
       <VBFixedString(10)> Dim ppnum As String             ' Primary Number
       <VBFixedString(8)> Dim psnum As String              ' Secondary Number
       <VBFixedString(3)> Dim prote As String              ' Rural Route
Number
       <VBFixedString(4)> Dim punit As String              ' Secondary
Number Unit
       <VBFixedString(2)> Dim ppre1 As String              ' First or Left
Pre-direction
       <VBFixedString(2)> Dim ppre2 As String              ' Second or
Right Pre-direction
       <VBFixedString(4)> Dim psuf1 As String              ' First or Left
Suffix
       <VBFixedString(4)> Dim psuf2 As String              ' Second or
Right Suffix
       <VBFixedString(2)> Dim ppst1 As String              ' First or Left
Post-direction
       <VBFixedString(2)> Dim ppst2 As String              ' Second or
Right Post-direction
       <VBFixedString(28)> Dim ppnam As String             ' Primary Name
       <VBFixedString(10)> Dim mpnum As String             ' Matched
Primary Number
       <VBFixedString(8)> Dim msnum As String              ' Matched
Secondary Number
       <VBFixedString(3)> Dim pmb As String                ' PMB Unit
Designator
       <VBFixedString(8)> Dim pmbnum As String             ' PMB Number
       Dim rsvd2 As Char()                                 ' Reserved for
Future User
       Dim foot As structFoot
       Dim ADDR_REC() As structADDR_REC                    ' Record Stack
       Dim rsvd4 As String                                 ' Reserved for
Future Use
   End Structure

   Structure structFoot
       Dim a As Char   ' Zip Corrected
       Dim b As Char   ' City/State Corrected
       Dim c As Char   ' Invalid City/State/Zip
       Dim d As Char   ' No Zip Assigned
       Dim e As Char   ' ZIP Assigned for Mult Response
       Dim f As Char   ' No Zip Available
       Dim g As Char   ' Part of Firm Moved to Address
       Dim h As Char   ' Secondary Number Missing
       Dim i As Char   ' Insufficient/Incorrect Data
       Dim j As Char   ' Dual Input - Used PO Box
       Dim k As Char   ' Dual Input - Used non-PO Box
       Dim l As Char   ' Del Addr Component Add/Del/Chg
       Dim m As Char   ' Street Name Spelling Changed
       Dim n As Char   ' Delivery Addr was Standardized
       Dim o As Char   ' Reserved for Future Use
       Dim p As Char   ' Better Delivery Addr Exists
       Dim q As Char   ' Reserved for Future Use
       Dim s As Char   ' Invalid Secondary Number
       Dim t As Char   ' Magnet Street
       Dim u As Char   ' Unofficial PO Name
       Dim v As Char   ' Unverifiable City/State
       Dim w As Char   ' Small Town Default
       Dim x As Char   ' Unique ZIP Code Default
       Dim y As Char   ' Reserved for Future Use
       Dim z As Char   ' ZIP Move Match
       Dim rsvd3 As Char()  ' Reserved for Future Use
   End Structure

When I do a call to Inquire, nothing changes in the structure.  The
structure should be populated with the corrected values, but the struct
remains unchanged.  Am I missing something?

Thank you for any assistance,