Marshaling Structures

Introduction

This tutorial will show all about structures that are:

1. Marshaling simple and complex structures
2. Passing Structures as Input/Output Parameters in functions

Background

You can combine data items of different types to create a structure. A structure associates one or more members with each other and with the structure itself. When you declare a structure, it becomes a composite data type, and you can declare variables of that type. A structure is a generalization of the user-defined type (UDT) supported by previous versions of Visual Basic. In addition to fields, structures can expose properties, methods, and events. A structure can implement one or more interfaces, and you can declare individual accessibility for each field.

Structures are useful when you want a single variable to hold several related pieces of information. For example, you might want to keep an employee's name, telephone extension, and salary together. You could use several variables for this information, or you could define a structure and use it for a single employee variable. The advantage of the structure becomes apparent when you have many employees and therefore many instances of the variable. The members can be of any data type, but you must include at least one nonshared variable or an event. You can also define Function procedures, properties, and events in a structure. You can define one property as the default property, provided it takes at least one argument. You can handle an event with a Shared Sub procedure.

You can specify the accessibility of a structure using the Public, Protected, Friend, or Private keyword, or you can let it default to Public. You must declare every member and specify accessibility for it. If you use the Dim statement without any keywords, the accessibility defaults to Public.

Furthermore, if using them in unmanaged code then you must specify the Layout the structure will use. I will use System.Runtime.InteropServices.Marshal class.

For information about Marshal class, see my article ‘Marshal Class Explained’ on this web site.

1. Marshaling Structures

   Simple Structure

   Let us consider a simple structure. We will copy its contents to a byte array as follow:

   Imports System.Runtime.InteropServices
   Imports System.Text

   From Structure to byte array:

   Private Structure Test
     Dim Var1 As Short
     Dim Var2 As Short
   End Structure
   
   ‘Start here
   Dim Tst As Test ' Stuctuer variable
   Dim ByteArray() As Byte
   ' initialize Structure (Dummmy  Values)
   Tst.Var1 = 911
   Tst.Var2 = 7
   Dim Ptr As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(Tst))
   ReDim ByteArray(Marshal.SizeOf(Tst) - 1)
   'now copy strcutre to Ptr pointer 
   Marshal.StructureToPtr(Tst, Ptr, False)
   Marshal.Copy(Ptr, ByteArray, 0, Marshal.SizeOf(Tst))
   ‘now use ByteArray ready for use 
   Marshal.FreeHGlobal(Ptr)

   Now reverse of above is as simple as coded below:

   From Byte array to Structure

   Let us consider a byte array. We will copy its content to a simple Structure Test as coded below:

   Private Structure Test 
      Dim Var1 As Short
      Dim Var2 As Short
   End Structure
   
   ' Function to Create structure must be called first
   Private Function BulidStr(ByVal Buff() As Byte, _
              ByVal MyType As System.Type) As Object
     Dim MyGC As GCHandle = GCHandle.Alloc(Buff,  GCHandleType.Pinned)
    ‘Marshals data from an unmanaged block of memory 
    'to a newly allocated managed object of the specified type.
     Dim Obj As Object = _
         Marshal.PtrToStructure(MyGC.AddrOfPinnedObject, MyType)
     Return Obj
    ‘Free GChandle to avoid memory leaks
     MyGC.Free()
   End Function
   
   ‘Start here
    Dim Str As String = "Adnan Samuel"
    Dim Tst As Test ‘ Stuctuer variable
    Dim ByteArray () As Byte, i As Int16
    ‘convert to bytes
    ByteArray = Encoding.Default.GetBytes(Str)’ Convert string to bytes
    ‘Call BulidStr method to create structure and 
    'copy byte array data to structure 
    Tst = BulidStr(ByteArray, Tst.GetType) 
    ‘Now use it and print it
    Debug.WriteLine(Tst.Var1.ToString)
    Debug.WriteLine(Tst.Var2.ToString)
   

   Complex Structures

   Let us consider following types of structures:

   • Embedded structure.
   • Structure that contain a pointer (which point another structure).

   Let us consider a structure (embedded) that contains a reference to other structures. We will copy its content to a byte array as:

   From Structure to byte array

   Private Structure SAFEARRAYBOUND
           Dim cElements As Integer
           Dim lLbound As Integer
       End Structure
       Private Structure SAFEARRAY2D
           Dim cDims As Integer
           Dim fFeatures As Integer
           Dim cbElements As Integer
           Dim cLocks As Integer
           Dim pvData As Integer
           ‘reference to above structure
           Dim Bounds() As SAFEARRAYBOUND 
       End Structure
   
    ' start code here
    Dim SA As SAFEARRAY2D
    Dim StrtoBy() As Byte 'check with integer array for better result
    'Must initialize array to hold data
    ReDim SA.Bounds(1)
    'first initalize your stucture here (I just put dummy values )
    SA.cDims = 11 : SA.fFeatures = 12 : SA.cbElements = 9 : SA.cLocks = 12
    SA.pvData = 6 : SA.Bounds(0).cElements = 15 : SA.Bounds(1).lLbound = 16
    ' first get size of SA in pointer
    Dim ptr As IntPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf(SA))
    'Must initialize before use 
    ReDim StrtoBy(Marshal.SizeOf(SA) - 1)
    Marshal.Copy(ptr, StrtoBy, 0, Marshal.SizeOf(SA))
    Marshal.FreeCoTaskMem(ptr)
    'now use byte array as you like

   Now reverse of this (byte to structure) is easy however you have to manually put elements in Bound array as shown below:

   From Byte array to Structure

       Private Structure SAFEARRAYBOUND
           Dim cElements As Integer
           Dim lLbound As Integer
       End Structure
       Private Structure SAFEARRAY2D
           Dim cDims As Byte
           Dim fFeatures As Byte
           Dim cbElements As Byte
           Dim cLocks As Byte
           Dim pvData As Byte
       End Structure
   
          ' start code here
           Dim Str As String = "Adnan Samuel (adahmed911@hotmail.com)"
           Dim Tst As SAFEARRAY2D  ' Structure variable
           Dim Bounds() As SAFEARRAYBOUND ‘define array manually
           Dim ByteArray() As Byte, i As Int16
           'Convert to bytes
           ' Convert string to bytes
           ByteArray = Encoding.Default.GetBytes(Str) 
           'Get array address and copy it to structure 
           Dim MyGC As GCHandle = _
               GCHandle.Alloc(ByteArray, GCHandleType.Pinned)
          ‘This will copy byte array to SAFEARRAY2D elements only (0 to 4)
           Tst = CType(Marshal.PtrToStructure(MyGC.AddrOfPinnedObject, _
                 GetType(SAFEARRAY2D)), SAFEARRAY2D)
           MyGC.Free()
           ‘now time for SAFEARRAYBOUND Structure members (5 & 6). 
           ReDim Bounds(1)
           ‘Show array values just to check         
           For i = 0 To 6 
               Debug.WriteLine(ByteArray(i))
           Next
           'copy remaining bits manually in bound array
           Bounds(0).cElements = ByteArray(5)
           Bounds(1).lLbound = ByteArray(6)
           'now use both structure ready fro use

   Let us consider a structure that contains a pointer to a second structure as a member. I will show you only how to handle it.

   <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> _
   Public Structure Emp1
           Public first As String
           Public last As String
   End Structure 
   
   <StructLayout(LayoutKind.Sequential)> _
   Public Structure Emp2
      Public Employee As IntPtr
      Public age As Integer
   End Structure 
        
   ‘Start code here
    Dim EmployeeName As Emp1
    EmployeeName.first = "Adnan"
    EmployeeName.last = "Samuel"
    Dim EmpAll As Emp2
    EmpAll.age = 26
    Dim buffer As IntPtr = Marshal.AllocCoTaskMem( _ 
              Marshal.SizeOf(EmployeeName))
    Marshal.StructureToPtr(EmployeeName, buffer, False)
    EmpAll.Employee = buffer
    Debug.WriteLine(ControlChars.CrLf & "Employee before call:")
    Debug.WriteLine("first = " & EmployeeName.first & vbCrLf & "last = " _
          & EmployeeName.last & vbCrLf & "age =" &                  
          EmpAll.age)
    'Now pass the Emp2 structure into unmanaged code (e.g. PinvokeLib.dll) _
    'as byRef then see the result
    ‘------
    Dim EmpRes As Emp1 = CType(Marshal.PtrToStructure(EmpAll.Employee, _
        GetType(Emp1)), Emp1)
    Marshal.FreeCoTaskMem(buffer)
    Debug.WriteLine(ControlChars.CrLf & "Employee after call:") 
    Debug.WriteLine("first = " & EmpRes.first & vbCrLf & "last = " & 
       EmpRes.last & vbCrLf & "age =" & EmpAll.age)

2. Passing Structures as Parameters

   • Input parameters
   • Input/Output parameters

   Let us consider a simple structure we pass it as ByVal in function:

    <StructLayout(LayoutKind.Sequential)> _
       Public Structure SECURITY_ATTRIBUTES
           Public nLength As Integer
           Public lpSecurityDescriptor As Integer
           Public bInheritHandle As Integer
       End Structure
   
       Public Declare Function CreateDirectory Lib "kernel32" _
            Alias "CreateDirectoryA" (ByVal lpPathName As String, _
            ByVal lpSecurityAttributes As SECURITY_ATTRIBUTES) As Boolean
           '' start code here
           Dim security As New SECURITY_ATTRIBUTES
           If CreateDirectory("c:\Adnan", security) Then
               MsgBox("Directory created.", MsgBoxStyle.Information)
           Else
               MsgBox("Directory not created.", MsgBoxStyle.Information)
           End If

   Let us consider an example in which we can pass structure as Input/Output parameter or simply ByRef parameter in a function.

   1. Start a new project a name it what you want then you have a form in a project.
   2. Now add a class name OSVersionInfo and add code below:

      Imports System.Runtime.InteropServices
      <StructLayout(LayoutKind.Sequential)> _
      Public Class OSVersionInfo
        Public OSVersionInfoSize As Integer
        Public majorVersion As Integer
        Public minorVersion As Integer
        Public buildNumber As Integer
        Public platformId As Integer
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _
        Public versionString As String
        Public ServicePackMajor As Integer
        Public ServicePackMinor As Integer
      End Class
      

   3. Now add another class and name it API and add code below:

      Imports System.Runtime.InteropServices
      Imports System.Runtime.InteropServices
      Public Class APIDeclare Ansi Function GetVersionEx _ 
          Lib "kernel32" Alias "GetVersionExA" _ 
          (<[In](), Out()> ByVal osvi As OSVersionInfo) As Boolean
      
        ‘Now you have option to use above function with I/O parameters or use 
        'below with ByRef but use one at a time
        ' Declare Ansi Function GetVersionEx2 Lib "kernel32" Alias _
        ' "GetVersionExA" (ByRef osvi As OSVersionInfo) As Boolean
      End Class

   4. Now add a button to Form1 and add the code.

      Private Sub Button1_Click(ByVal sender As System.Object, ByVal _ 
             e As System.EventArgs) Handles Button1.Click
       Dim versionInfo As New OSVersionInfo
       versionInfo.OSVersionInfoSize = Marshal.SizeOf(versionInfo)
       GetVersionEx(versionInfo)
       Debug.WriteLine("Major Version is: " & versionInfo.majorVersion)
       Debug.WriteLine("Minor Version is: " _
            & versionInfo.minorVersion.ToString)
       Debug.WriteLine("Bulid Number is: " _
            & versionInfo.buildNumber.ToString())
       Debug.WriteLine("Service Pack: " _
            & versionInfo.ServicePackMajor.ToString())
      
      End Sub

I am always willing to help, so if you have any questions, suggestions about my article, feel free to email me. You can also reach me on msn messenger with screen name “Maxima”.

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