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Passing Structures

Many unmanaged functions expect you to pass, as a parameter to the function, members of structures (user-defined types in Visual Basic) or members of classes that are defined in managed code. When passing structures or classes to unmanaged code using platform invoke, you must provide additional information to preserve the original layout and alignment. This topic introduces the StructLayoutAttribute attribute, which you use to define formatted types. For managed structures and classes, you can select from several predictable layout behaviors supplied by the LayoutKind enumeration.

Central to the concepts presented in this topic is an important difference between structure and class types. Structures are value types and classes are reference types — classes always provide at least one level of memory indirection (a pointer to a value). This difference is important because unmanaged functions often demand indirection, as shown by the signatures in the first column of the following table. The managed structure and class declarations in the remaining columns show the degree to which you can adjust the level of indirection in your declaration.

Unmanaged signature

Managed declaration:

no indirection

struct MyStruct(…);

Managed declaration:

one level of indirection

class MyStruct(…);

DoWork(MyStruct x);

Demands zero levels of indirection.

DoWork(ByVal x As MyStruct)

Adds zero levels of indirection.

Not possible because there is already one level of indirection.

DoWork(MyStruct* x);

Demands one level of indirection.

DoWork(ByRef x As MyStruct)

Adds one level of indirection.

DoWork(ByVal x As MyStruct)

Adds zero levels of indirection.

DoWork(MyStruct** x);

Demands two levels of indirection.

Not possible because ByRef ByRef cannot be used.

DoWork(ByRef x As MyStruct)

Adds one level of indirection.

The table describes the following guidelines for platform invoke declarations:

  • Use a structure passed by value when the unmanaged function demands no indirection.

  • Use either a structure passed by reference or a class passed by value when the unmanaged function demands one level of indirection.

  • Use a class passed by reference when the unmanaged function demands two levels of indirection.

The following example shows how to define the Point and Rect structures in managed code, and pass the types as parameter to the PtInRect function in the User32.dll file. PtInRect has the following unmanaged signature:

BOOL PtInRect(const RECT *lprc, POINT pt);

Notice that you must pass the Rect structure by reference, since the function expects a pointer to a RECT type.

Imports System.Runtime.InteropServices

<StructLayout(LayoutKind.Sequential)> Public Structure Point
    Public x As Integer
    Public y As Integer
End Structure

Public Structure <StructLayout(LayoutKind.Explicit)> Rect
    <FieldOffset(0)> Public left As Integer
    <FieldOffset(4)> Public top As Integer
    <FieldOffset(8)> Public right As Integer
    <FieldOffset(12)> Public bottom As Integer
End Structure

Class Win32API    
    Declare Auto Function PtInRect Lib "user32.dll" _
    (ByRef r As Rect, p As Point) As Boolean
End Class

You can pass members of a class to an unmanaged DLL function, as long as the class has a fixed member layout. The following example demonstrates how to pass members of the MySystemTime class, which are defined in sequential order, to the GetSystemTime in the User32.dll file. GetSystemTime has the following unmanaged signature:

void GetSystemTime(SYSTEMTIME* SystemTime);

Unlike value types, classes always have at least one level of indirection.

Imports System.Runtime.InteropServices
Imports Microsoft.VisualBasic

<StructLayout(LayoutKind.Sequential)> Public Class MySystemTime
    Public wYear As Short
    Public wMonth As Short
    Public wDayOfWeek As Short 
    Public wDay As Short
    Public wHour As Short
    Public wMinute As Short
    Public wSecond As Short
    Public wMiliseconds As Short
End Class

Public Class Win32
    Declare Auto Sub GetSystemTime Lib "Kernel32.dll"(sysTime _
        As MySystemTime)
    Declare Auto Function MessageBox Lib "User32.dll"(hWnd As IntPtr, _
        txt As String, caption As String, Typ As Integer) As Integer
End Class

Public Class TestPlatformInvoke    
    Public Shared Sub Main()
        Dim sysTime As New MySystemTime()
        Win32.GetSystemTime(sysTime)

        Dim dt As String
        dt = "System time is:" & ControlChars.CrLf & _
              "Year: " & sysTime.wYear & _
              ControlChars.CrLf & "Month: " & sysTime.wMonth & _
              ControlChars.CrLf & "DayOfWeek: " & sysTime.wDayOfWeek & _
              ControlChars.CrLf & "Day: " & sysTime.wDay
        Win32.MessageBox(IntPtr.Zero, dt, "Platform Invoke Sample", 0)      
    End Sub
End Class

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