Using Platform Invoke

Introduction

In this article, I will summarize some of the points involved in the usage of PInvoke.

PInvoke is the mechanism by which .NET languages can call unmanaged functions in DLLs. This is especially useful for calling Windows API functions that aren’t encapsulated by the .NET Framework classes, as well as for other third-party functions provided in DLLs.

PInvoke differs its usage while used in Visual C# or Managed C++ compared with VB because those languages can use pointers or specify unsafe code, which VB cannot.

Using PInvoke in VB

There are mainly two ways in which you can use PInvoke in VB.

1. Using Declare statement
   VB

   Declare Auto Function MyMessageBox Lib "user32.dll" Alias _
    "MessageBox" (ByVal hWnd as Integer, ByVal msg as String, _ 
    ByVal Caption as String, ByVal Tpe as Integer) As Integer
   
   Sub Main()
       MyMessageBox(0, "Hello World !!!", "Project  Title", 0)
   End Sub

   • Auto/Ansi/Unicode: Character encoding type. Use Auto and leave it up to the compiler to decide.
   • Lib: Library name. Must be in quotes.
   • Name & Alias name for function: If the DLL function name is a VB keyword, then it is needed.
2. Using DllImport
   VB

    Imports System.Runtime.InteropServices
    
   <DllImport("User32.dll")> Public Shared Function _ 
     MessageBox(ByVal hWnd As Integer, _ 
     ByVal txt As String, ByVal caption As String, _ 
     ByVal typ As Integer) As Integer
   End Function
   
       Sub Main()
           MessageBox(0, "Imported Hello World !!!", "Project Title", 0)
       End Sub

The Declare statement has been provided for backward compatibility with VB 6.0. Actually VB.NET compiler converts Declare to DllImport, but if you need to use any advance options as mentioned below, you have to go for DllImport.

When using DllImport, the function from the DLL is implemented as an empty function with the name, arguments and return type. It has the DllImport attribute, which specifies the name of the DLL containing the function. The runtime will search for it looking in the current directory, the Windows System32 directory, and then in the path. (If name is a keyword then use square braces.)

Following is the list of parameters used with DllImport.

Parameter	Description
BestFitMapping	Marshaler will find a best match for chars that can't be mapped between ANSI & Unicode when enabled. Defaults to True.
CallingConvention	The calling convention of a DLL entry point. Defaults to stdcall.
CharSet	Indicates how to marshal string data and which entry point to choose when both ANSI and Unicode versions are available. Defaults to Charset.Auto.
EntryPoint	This specifies the name or ordinal value of the entry point to be used in the DLL. If not given, function name is used as entry point.
ExactSpelling	It controls whether the interop marshaler will perform name mapping.
PreserveSig	Specifies whether to preserve function signature while conversion.
SetLastError	Whether the method will call Win32 SetLastError API or not. To retrieve the error, use Marshal.GetLastWin32Error.
ThrowOnUnmappableChar	If false, unmappable characters are replaced by a question mark (?). If true, an exception is thrown when an unmappable character is encountered.

Using PInvoke in C#

Unlike VB, C# does not have the Declare keyword, so we need to use DllImport in C# and Managed C++.

[DllImport("user32.dll"]
  public static extern int MessageBoxA(
          int h, string m, string c, int type);

Here the function is declared as static because function is not instance dependent, and extern because C# compiler should be notified not to expect implementation.

C# also provides a way to work with pointers. C# code that uses pointers is called unsafe code and requires the use of keywords: unsafe and fixed. If unsafe flag is not used, it will result in compiler error.

Any operation in C# that involves pointers must take place in an unsafe context. We can use the unsafe keyword at class, method and block levels as shown below.

public unsafe class myUnsafeClass
{
      //This class can freely use unsafe code.
}

public class myUnsafeClass
{
      //This class can NOT use unsafe code.
     public unsafe void myUnsafeMethod
     {
            // this method can use unsafe code 
      }
}

public class myUnsafeClass
{
      //This class can NOT use unsafe code.
     public void myUnsafeMethod
     {
            // this method too can NOT use unsafe code 
            unsafe 
            {
                 // Only this block can use unsafe code.
            }
      }
}

• stackalloc

  The stackalloc keyword is sometimes used within unsafe blocks to allow allocating a block of memory on the stack rather than on the heap.

• fixed & pinning

  GC moves objects in managed heap when it compacts memory during a collection.

  If we need to pass a pointer to a managed object to an unmanaged function, we need to ensure that the GC doesn’t move the object while its address is being used through the pointer. This process of fixing an object in memory is called pinning, and it’s accomplished in C# using the fixed keyword.

• MarshalAs

  MarshalAs attribute can be used to specify how data should be marshaled between managed and unmanaged code when we need to override the defaults. When we are passing a string to a COM method, the default conversion is a COM BSTR; when we are passing a string to a non-COM method, the default conversion is C- LPSTR. But if you want to pass a C-style null-terminated string to a COM method, you will need to use MarshalAs to override the default conversion.

  So far we have seen simple data types. But some of the functions need structures to be passed, which we have to handle differently from simple data types.

• StructLayout

  We can define a managed type that is the equivalent of an unmanaged structure. The problem with marshaling such types is that the common language runtime controls the layout of managed classes and structures in memory. The StructLayout Attribute allows a developer to control the layout of managed types. Possible values for the StructLayout are Auto, Explicit and Sequential. Charset, Pack and Size are the optional parameters which can be used with the StructLayout attribute.

Callback functions and passing arrays as parameters involve some more complications and can be the subject for the next article.

Parameter type mapping

One of the severe problems with using Platform Invoke is deciding which .NET type to use when declaring the API function. The following table will summarize the .NET equivalents of the most commonly used Windows data types.

Windows Data Type	.NET Data Type
BOOL, BOOLEAN	Boolean or Int32
BSTR	String
BYTE	Byte
CHAR	Char
DOUBLE	Double
DWORD	Int32 or UInt32
FLOAT	Single
HANDLE (and all other handle types, such as HFONT and HMENU)	IntPtr, UintPtr or HandleRef
HRESULT	Int32 or UInt32
INT	Int32
LANGID	Int16 or UInt16
LCID	Int32 or UInt32
LONG	Int32
LPARAM	IntPtr, UintPtr or Object
LPCSTR	String
LPCTSTR	String
LPCWSTR	String
LPSTR	String or StringBuilder*
LPTSTR	String or StringBuilder
LPWSTR	String or StringBuilder
LPVOID	IntPtr, UintPtr or Object
LRESULT	IntPtr
SAFEARRAY	.NET array type
SHORT	Int16
TCHAR	Char
UCHAR	SByte
UINT	Int32 or UInt32
ULONG	Int32 or UInt32
VARIANT	Object
VARIANT_BOOL	Boolean
WCHAR	Char
WORD	Int16 or UInt16
WPARAM	IntPtr, UintPtr or Object

*As the string is an immutable class in .NET, they aren't suitable for use as output parameters. So a StringBuilder is used instead.