DLLImport and <[In](), Out()>

[ehelin]

Hi:

 

Can anyone shed any light on the <[in](), Out()> tags sometimes associated with DLL calls?

 

For example, look at the hypothetical call below...in many of the examples I see on the internet, I see these in/out tags. I know that they specify the direction of the variable, but how do you determine whether the argument for a dll is in/out or some variation of both?

 

<DllImport("file.dll", EntryPoint:="#1")> _

Private Shared Function DSMparent(<[in](), Out()> _

ByVal arg1 As String, _

ByVal arg2 As String, _

ByVal arg3 As String, _

<[in](), Out()> ByVal arg4 As String) As String

End Function

 

A real example is

 

<DllImport("gdiplus.dll", ExactSpelling:=True, CharSet:=CharSet.Unicode)>

Friend Shared Function GdipSaveImageToFile(

ByVal image As IntPtr,

ByVal filename As String,

<[in]()> ByRef clsid As Guid,

ByVal encparams As IntPtr)

As Integer

End Function

[dynamic_sysop]

if you look on msdn at the way api's are made up, you may get a better insight

 

[in] would be a pointer , string , int32 etc.. you are injecting in to the api call

[Out] would be a pointer etc... that you are receiving

[in] [Out] would work as both

 

eg: take a look at the FindWindow Api ( for C++ ) on msdn, the description of the Parameters specifies you are injecting a value in to the call.

 

HWND FindWindow(

 

LPCTSTR

lpClassName

, LPCTSTR

lpWindowName

);

Parameters

 

lpClassName

[in]

Pointer to a null-terminated string that specifies the class name or a class atom created by a previous call to the

RegisterClass

or

RegisterClassEx

function. The atom must be in the low-order word of

lpClassName

; the high-order word must be zero. If

lpClassName

points to a string, it specifies the window class name. The class name can be any name registered with

RegisterClass

or

RegisterClassEx

, or any of the predefined control-class names.

 

If

lpClassName

is NULL, it finds any window whose title matches the

lpWindowName

parameter.

 

lpWindowName

[in]

Pointer to a null-terminated string that specifies the window name (the window's title). If this parameter is NULL, all window names match.

where as, the GetWindowText api holds an [Out] value, where it receives a string...

int GetWindowText(

 

HWND

hWnd

, LPTSTR

lpString

, int

nMaxCount

);

Parameters

 

hWnd

[in]

Handle to the window or control containing the text.

lpString

[out]

Pointer to the buffer that will receive the text. If the string is as long or longer than the buffer, the string is truncated and terminated with a NULL character.

nMaxCount

[in]

Specifies the maximum number of characters to copy to the buffer, including the NULL character. If the text exceeds this limit, it is truncated.

[VagabondSW]

In Visual Basic, the Directional Attributes will determine whether you use ByRef or ByVal when defining arguments for a field or function.

 

http://msdn.microsoft.com/library/en-us/cpguide/html/interopmarshalrules2.gif