bool pointer from C++ to C#

Question

I want to use a function in a C++ DLL in my C# application using DLLImport:

BOOL IsEmpty( DWORD KeyID, BOOL *pFlag )

I tried many combinations but in vain, like:

public extern static bool IsEmpty(int KeyID, ref bool pFlag);

The method returns false (that means an error).

Any idea how to do that?

Minor note, DWORD is an unsigned 32-bit type. The corresponding type in C# is uint, not int. — cdhowie
– cdhowie, Commented Oct 3, 2012 at 15:09
bool is the standard C++ type. BOOL is a MFC type. they are not the same. — Daniel Daranas
– Daniel Daranas, Commented Oct 3, 2012 at 15:11

Community · Accepted Answer · 2017-05-23 10:24:16Z

4

To quote "Willy" (with amendments):

Beware the booleans!

Win32 defines different versions of booleans.

1) BOOL used by most Win32 API's, is ~~an unsigned int~~ a signed int (4 bytes)

2) BOOLEAN is a single byte, only used by a few win32 API's!!

3) and C/C++ has it's builtin 'bool' which is a single byte

...and to add what @tenfour pointed out:

4) the even more bizarre VARIANT_BOOL

typedef short VARIANT_BOOL;
#define VARIANT_TRUE ((VARIANT_BOOL)-1)
#define VARIANT_FALSE ((VARIANT_BOOL)0)

The signed or unsigned nature shouldn't matter for BOOL, as the only "false" pattern is 0. So try treating it as a 4 byte quantity...however you interface with a DWORD may be satisfactory, (I've not dealt with Windows 64-bit conventions.)

edited May 23, 2017 at 10:24

CommunityBot

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answered Oct 3, 2012 at 15:12

HostileFork says dont trust SE

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2 Comments

cdhowie Over a year ago

I don't believe that the size of BOOL nor DWORD changes on 64-bit Windows.

tenfour Over a year ago

I'd add the even more bizarre VARIANT_BOOL where VARIANT_FALSE is -1.

BigBoss · Accepted Answer · 2012-10-03 15:12:13Z

1

BOOL in Win32 is a typedef of int, so you should just change bool to Int32, so the definition is int IsEmpty(uint KeyID, ref int pFlag)

answered Oct 3, 2012 at 15:12

BigBoss

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Comments

Manuel Amstutz · Accepted Answer · 2012-10-03 15:18:32Z

1

because in c++ BOOL is defined as int. You should use

    [return: System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.Bool)]
    public static extern  bool IsEmpty(uint KeyID, ref int pFlag) ;

answered Oct 3, 2012 at 15:18

Manuel Amstutz

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1 Comment

HostileFork says dont trust SE Over a year ago

To be more precise: BOOL is defined inside the Win32 API header files (not any C++ standard, though C++ added a bool that did not exist in C). MSDN does however say it's a signed int from WinDef.h...not an unsigned as from the source I quoted. Although signed/unsigned technically should not matter as only 0 is FALSE and that has the same bit pattern regardless. :-/

user1717487 · Accepted Answer · 2012-10-04 07:58:23Z

0

Thank you for your help!

finally this works for me

public extern static int IsEmpty( int KeyID, int[] pFlag)

answered Oct 4, 2012 at 7:58

user1717487

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