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OutArrayOfStructs Sample

  • Article

This sample demonstrates how to pass an array of structures that contains integers and strings as Out parameters to an unmanaged function. It is assumed that the callee allocates memory for the array. The C# sample code demonstrates how to call this function in two different ways: using methods of the Marshal class and using unsafe code. The Microsoft .NET Framework SDK includes the complete Visual Basic .NET and C# versions of this sample in Samples\Technologies\Interop\Platform-Invoke.

The OutArrayOfStructs sample uses the following unmanaged function, shown with its original function declaration:

  • TestOutArrayOfStructs exported from PinvokeLib.dll.

    void TestOutArrayOfStructs(int* pSize, MYSTRSTRUCT2** ppStruct);

PinvokeLib.dll is a custom unmanaged library that contains an implementation for the previously listed function and the MYSTRSTRUCT2 structure. The structure contains the following elements:

typedef struct _MYSTRSTRUCT2
{
   char* buffer;
   UINT size; 
} MYSTRSTRUCT2;

In this sample, the MyStruct class contains a string object of ANSI characters. The CharSet field specifies ANSI format. MyUnsafeStruct, which appears only in the C# sample, is a structure containing an IntPtr type instead of a string.

The LibWrap class contains the overloaded TestOutArrayOfStructs prototype method. If a method declares a pointer as a parameter, the class should be marked with the unsafe keyword. Because Visual Basic .NET cannot use unsafe code, the overloaded method, unsafe modifier, and the MyUnsafeStruct structure are unnecessary.

The App class implements the UsingMarshal method, which performs all the tasks necessary to pass the array. The array is marked with the out (ByRef in Visual Basic) keyword to indicate that data passes from callee to caller. The implementation uses the following Marshal class methods:

As previously mentioned, C# allows unsafe code and Visual Basic .NET does not. In the C# sample, UsingUnsafe is an alternative method implementation that uses pointers instead of the Marshal class to pass back the array containing the MyUnsafeStruct structure.

Declaring Prototypes

' Declares a class member for each structure element.
< StructLayout( LayoutKind.Sequential, CharSet:=CharSet.Ansi )> _
Public Class MyStruct 
   Public buffer As String 
   Public someSize As Integer
End Class 'MyStruct

Public Class LibWrap
' Declares a managed prototype for the unmanaged function.
   Declare Sub TestOutArrayOfStructs Lib "..\\LIB\\PinvokeLib.dll" ( _
      ByRef arrSize As Integer, ByRef outArray As IntPtr )
End Class 'LibWrap
[C#]
// Declares a class member for each structure element.
[ StructLayout( LayoutKind.Sequential, CharSet=CharSet.Ansi )]
public class MyStruct 
{
   public String buffer;
   public int size;
}
// Declares a structure with a pointer.
[ StructLayout( LayoutKind.Sequential )]
public struct MyUnsafeStruct 
{
   public IntPtr buffer;
   public int size;
}

public unsafe class LibWrap
{
   // Declares managed prototypes for the unmanaged function.
   [ DllImport( "..\\LIB\\PinvokeLib.dll" )]
   public static extern void TestOutArrayOfStructs( out int size, 
      out IntPtr outArray );
   [ DllImport( "..\\LIB\\PinvokeLib.dll" )]
   public static extern void TestOutArrayOfStructs( out int size, 
   MyUnsafeStruct** outArray );
}

Calling Functions

Public Class App
   Public Shared Sub Main()
      Console.WriteLine( ControlChars.CrLf & "Using marshal class" & _
ControlChars.CrLf )
      UsingMarshal()
      'Visual Basic .NET cannot use unsafe code.
   End Sub 'Main
   
   Public Shared Sub UsingMarshal()   
      Dim arrSize As Integer
      Dim outArray As IntPtr
      LibWrap.TestOutArrayOfStructs( arrSize, outArray )
      Dim manArray(arrSize - 1) As MyStruct
      Dim current As IntPtr = outArray
      Dim i As Integer
      
      For i = 0 To arrSize - 1
      
         manArray(i) = New MyStruct()
         Marshal.PtrToStructure( current, manArray(i))
         
         Marshal.DestroyStructure( current, GetType( MyStruct )) 
         current = IntPtr.op_explicit( current.ToInt32() _
           + Marshal.SizeOf( manArray(i) ))
         
         Console.WriteLine( "Element {0}: {1} {2}", i, manArray(i)._
           buffer, manArray(i).someSize )
      Next i
      Marshal.FreeCoTaskMem( outArray )
   End Sub 'UsingMarshal
End Class 'App

[C#]
public class App
{
   public static void Main()
   {
      Console.WriteLine( "\nUsing marshal class\n" );
      UsingMarshal();
      Console.WriteLine( "\nUsing unsafe code\n" );
      UsingUnsafe();
   }
   
   public static void UsingMarshal()   
   {
      int size;
      IntPtr outArray;
      LibWrap.TestOutArrayOfStructs( out size, out outArray );
      MyStruct[] manArray = new MyStruct[ size ];
      IntPtr current = outArray;
      for( int i = 0; i < size; i++ )
      {
         manArray[ i ] = new MyStruct();
         Marshal.PtrToStructure( current, manArray[ i ]);
         
         //Marshal.FreeCoTaskMem( (IntPtr)Marshal.ReadInt32( current ));
         Marshal.DestroyStructure( current, typeof(MyStruct) );
         current = (IntPtr)((int)current + 
            Marshal.SizeOf( manArray[ i ] ));
         
         Console.WriteLine( "Element {0}: {1} {2}", i, 
            manArray[ i ].buffer, manArray[ i ].size );
      }
      Marshal.FreeCoTaskMem( outArray );
   }
   
   public static unsafe void UsingUnsafe()
   {
      int size;
      MyUnsafeStruct* pResult;
      LibWrap.TestOutArrayOfStructs( out size, &pResult );
      MyUnsafeStruct* pCurrent = pResult;
      for( int i = 0; i < size; i++, pCurrent++ )
      {
         Console.WriteLine( "Element {0}: {1} {2}", i, 
            Marshal.PtrToStringAnsi( pCurrent->buffer ), pCurrent->size );
         Marshal.FreeCoTaskMem( pCurrent->buffer );
      }
      Marshal.FreeCoTaskMem( (IntPtr)pResult );
   }
}

See Also

Marshaling Classes, Structures, and Unions | Platform Invoke Data Types | Creating Prototypes in Managed Code