OSInfo Sample
This sample demonstrates how to pass either a formatted class by value or a structure by reference as a parameter to an unmanaged function that expects a structure with an embedded character buffer. The Microsoft .NET Framework SDK includes the complete Visual Basic .NET and C# versions of this sample in Samples\Technologies\Interop\Platform-Invoke.
The OSInfo sample uses the following unmanaged function, shown with its original function declaration:
GetVersionEx exported from Kernel32.dll.
// BOOL GetVersionEx(LPOSVERSIONINFO lpVersionInfo);
The original structure passed to the function contains the following elements:
typedef struct _OSVERSIONINFO
{
DWORD dwOSVersionInfoSize;
DWORD dwMajorVersion;
DWORD dwMinorVersion;
DWORD dwBuildNumber;
DWORD dwPlatformId;
TCHAR szCSDVersion[ 128 ];
} OSVERSIONINFO;
In this sample, the OSVersionInfo
class and the OSVersionInfo2
structure produce the same results when passed to the unmanaged function. The MarshalAsAttribute attribute sets the UnmanagedType enumeration value to ByValTStr, which is used to identify the inline, fixed-length character array that appears within the unmanaged structure.
The LibWrap
class contains two prototypes: GetVersionEx
passes the class as a parameter and GetVersionEx2
passes a structure as a parameter. By applying the InAttribute and OutAttribute attributes explicitly, the sample ensures that OSVersionInfo
is marshaled as an In/Out parameter and the caller can see the changes marshaled back. (For performance, the default directional attribute for a class is In, which prevents the caller from seeing the changes marshaled back.)
The OSVersionInfo2
structure, which is normally passed by value, is declared with the ref (ByRef in Visual Basic) keyword and is passed by reference. The Marshal.SizeOf method determines the size, in bytes, of the unmanaged structure.
Declaring Prototypes
< StructLayout( LayoutKind.Sequential )> _
Public Class OSVersionInfo
Public OSVersionInfoSize As Integer
...
< MarshalAs( UnmanagedType.ByValTStr, SizeConst := 128 )> _
Public versionString As String
End Class 'OSVersionInfo
< StructLayout( LayoutKind.Sequential )> _
Public Structure OSVersionInfo2
Public OSVersionInfoSize As Integer
...
< MarshalAs( UnmanagedType.ByValTStr, SizeConst := 128 )> _
Public versionString As String
End Structure 'OSVersionInfo2
Public Class LibWrap
Declare Ansi Function GetVersionEx Lib "kernel32" Alias _
"GetVersionExA" ( <[In], Out> ByVal osvi As OSVersionInfo ) As Boolean
Declare Ansi Function GetVersionEx2 Lib "kernel32" Alias _
"GetVersionExA" ( ByRef osvi As OSVersionInfo2 ) As Boolean
End Class 'LibWrap
[C#][ StructLayout( LayoutKind.Sequential )]
public class OSVersionInfo
{
public int OSVersionInfoSize;
...
[ MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )]
public String versionString;
}
[ StructLayout( LayoutKind.Sequential )]
public struct OSVersionInfo2
{
public int OSVersionInfoSize;
...
[ MarshalAs( UnmanagedType.ByValTStr, SizeConst=128 )]
public String versionString;
}
public class LibWrap
{
[ DllImport( "kernel32" )]
public static extern bool GetVersionEx( [In, Out] OSVersionInfo osvi );
[ DllImport( "kernel32", EntryPoint="GetVersionEx" )]
public static extern bool GetVersionEx2( ref OSVersionInfo2 osvi );
}
Calling Functions
Public Class App
Public Shared Sub Main()
Console.WriteLine( ControlChars.CrLf + "Passing OSVersionInfo _
as class" )
Dim osvi As New OSVersionInfo()
osvi.OSVersionInfoSize = Marshal.SizeOf( osvi )
LibWrap.GetVersionEx( osvi )
Console.WriteLine( "Class size: {0}", osvi.OSVersionInfoSize )
...
Console.WriteLine( ControlChars.CrLf + "Passing OSVersionInfo _
as struct" )
Dim osvi2 As New OSVersionInfo2()
osvi2.OSVersionInfoSize = Marshal.SizeOf( osvi2 )
LibWrap.GetVersionEx2( osvi2 )
Console.WriteLine( "Struct size: {0}", osvi2.OSVersionInfoSize )
...
End Sub 'Main
End Class 'App
[C#]public class App
{
public static void Main()
{
Console.WriteLine( "\nPassing OSVersionInfo as class" );
OSVersionInfo osvi = new OSVersionInfo();
osvi.OSVersionInfoSize = Marshal.SizeOf( osvi );
LibWrap.GetVersionEx( osvi );
Console.WriteLine( "Class size: {0}", osvi.OSVersionInfoSize );
...
Console.WriteLine( "\nPassing OSVersionInfo as struct" );
OSVersionInfo2 osvi2 = new OSVersionInfo2();
osvi2.OSVersionInfoSize = Marshal.SizeOf( osvi2 );
LibWrap.GetVersionEx2( ref osvi2 );
Console.WriteLine( "Struct size: {0}", osvi2.OSVersionInfoSize );
...
}
}
See Also
Marshaling Strings | Platform Invoke Data Types | Default Marshaling for Strings | Creating Prototypes in Managed Code