Marshal.PtrToStructure skipping bytes

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  • spamacon@gmail.com
    #1

    Marshal.PtrToStructure skipping bytes

    Hello,
    I have a strange situation using .Net FW 1.1. I want to use
    Marshal.PtrToSt ructure to fill the structure below. The first 3 fields
    get filled correctly: ulStruct describes how big the structure is in
    bytes (120 bytes, assuming 32-bit IntPtrs), pWmSnapshot gets 0, and
    usNumWmSnapshot s gets 0. The next field, pICView, is a pointer to
    another structure, and gets filled with some garbage location, which is
    actually the lower 2 bytes of the real location, and the 2 bytes of the
    next field, usNumViews. The usNumViews field is then filled with the 2
    bytes that were supposed to go into usItemType, and all subsequent
    fields are off by those 2 bytes.

    If I manually extract the structure from the pointer, using
    Marshal.ReadInt 32, ReadInt16, ReadIntPtr, and PtrToStringAnsi ,
    everything is fine. That is, if I do Marshal.ReadInt Ptr(ptr_to_stru ct,
    10) it reads the correct location for pICView.

    I've tried making the pICView field an integer (MarshalAs U4), and it
    skips the same 2 bytes.

    I haven't had any other problems using PtrToStructure on other
    structures, just this one hiccup. Any ideas?

    Thanks,
    Adam

    Structure:

    <StructLayout(L ayoutKind.Seque ntial, CharSet:=CharSe t.Ansi)_
    Public Structure SNAPSHOTSTRUCT
    <MarshalAs(Unma nagedType.U4)Pu blic ulStruct As Integer
    Public pWmSnapshot As IntPtr 'PWMSNAPSHOTSTR UCT
    <MarshalAs(Unma nagedType.U2)Pu blic usNumWmSnapshot s As Short
    Public pICView As IntPtr 'PICVIEWSTRUCT
    <MarshalAs(Unma nagedType.U2)Pu blic usNumViews As Short
    <MarshalAs(Unma nagedType.U2)Pu blic usItemType As Short
    <MarshalAs(Unma nagedType.U4)Pu blic ulOpenStatus As Integer
    <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=17)P ublic
    szItemID As String
    <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
    useridCheckout As String
    <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
    tsCreate As String
    <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
    tsModify As String
    End Structure

    Tags: None
    • spamacon@gmail.com
      #2
      Re: Marshal.PtrToSt ructure skipping bytes

      Well, to respond to my own extremely popular topic, I found the
      problem. The combination of integers, integer pointers, and shorts
      (2-bytes) required me to set the packing on the structure to 2, instead
      of the default 8. I can only surmise that the 3rd field, which is a
      short, was properly getting only the 2 bytes, but was padded out to the
      next 2. Set the packing attribute on the structure solves the problem.

      spamacon@gmail. com wrote:
      Hello,
      I have a strange situation using .Net FW 1.1. I want to use
      Marshal.PtrToSt ructure to fill the structure below. The first 3 fields
      get filled correctly: ulStruct describes how big the structure is in
      bytes (120 bytes, assuming 32-bit IntPtrs), pWmSnapshot gets 0, and
      usNumWmSnapshot s gets 0. The next field, pICView, is a pointer to
      another structure, and gets filled with some garbage location, which is
      actually the lower 2 bytes of the real location, and the 2 bytes of the
      next field, usNumViews. The usNumViews field is then filled with the 2
      bytes that were supposed to go into usItemType, and all subsequent
      fields are off by those 2 bytes.
      >
      If I manually extract the structure from the pointer, using
      Marshal.ReadInt 32, ReadInt16, ReadIntPtr, and PtrToStringAnsi ,
      everything is fine. That is, if I do Marshal.ReadInt Ptr(ptr_to_stru ct,
      10) it reads the correct location for pICView.
      >
      I've tried making the pICView field an integer (MarshalAs U4), and it
      skips the same 2 bytes.
      >
      I haven't had any other problems using PtrToStructure on other
      structures, just this one hiccup. Any ideas?
      >
      Thanks,
      Adam
      >
      Structure:
      >
      <StructLayout(L ayoutKind.Seque ntial, CharSet:=CharSe t.Ansi)_
      Public Structure SNAPSHOTSTRUCT
      <MarshalAs(Unma nagedType.U4)Pu blic ulStruct As Integer
      Public pWmSnapshot As IntPtr 'PWMSNAPSHOTSTR UCT
      <MarshalAs(Unma nagedType.U2)Pu blic usNumWmSnapshot s As Short
      Public pICView As IntPtr 'PICVIEWSTRUCT
      <MarshalAs(Unma nagedType.U2)Pu blic usNumViews As Short
      <MarshalAs(Unma nagedType.U2)Pu blic usItemType As Short
      <MarshalAs(Unma nagedType.U4)Pu blic ulOpenStatus As Integer
      <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=17)P ublic
      szItemID As String
      <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
      useridCheckout As String
      <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
      tsCreate As String
      <MarshalAs(Unma nagedType.ByVal TStr, sizeconst:=27)P ublic
      tsModify As String
      End Structure

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