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.
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
MyMessageBox(0, "Hello World !!!", "Project Title", 0)
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.
<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
MessageBox(0, "Imported Hello World !!!", "Project Title", 0)
Declare statement has been provided for backward compatibility with VB 6.0. Actually VB.NET compiler converts
DllImport, but if you need to use any advance options as mentioned below, you have to go for
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
|Marshaler will find a best match for chars that can't be mapped between ANSI & Unicode when enabled. Defaults to |
|The calling convention of a DLL entry point. Defaults to |
|Indicates how to marshal string data and which entry point to choose when both ANSI and Unicode versions are available. Defaults to |
|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.|
|It controls whether the interop marshaler will perform name mapping.|
|Specifies whether to preserve function signature while conversion.|
|Whether the method will call Win32 |
SetLastError API or not. To retrieve the error, use
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++.
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 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
public class myUnsafeClass
public unsafe void myUnsafeMethod
public class myUnsafeClass
public void myUnsafeMethod
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
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.
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
Size are the optional parameters which can be used with the
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|
HANDLE (and all other handle types, such as
*As the string is an immutable class in .NET, they aren't suitable for use as output parameters. So a
StringBuilder is used instead.