COM: Passing struct from C# to C++

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Hi there,

what is the best way to pass a user defined struct from an C# assembly to an unmanaged C++ .ocx via COM? Especially the dispatch_map(?) statement:

DISP_FUNCTION_ID(Foo, "Bar", dispidBar, Bar, VT_BOOL, VTS_UI1 VTS_UI1 VTS_UI1 VTS_UI1 [mycustomstruct])

Just got the project from someone who left without any useable documentation. Also i'm not familiar with com, dcom whatsoever. Any useful links would be appreciated. So far i found SafeArrays, VT_Record, ... but not really sure which one and how to use those. Same goes for dispatch maps ...

Thanks in advance,
crkn

Posted 27-Nov-12 6:36am

crkn

Updated 27-Nov-12 6:37am

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Sergey Alexandrovich Kryukov 27-Nov-12 12:49pm

Why COM? What's the roles of both components?
--SA

3 solutions

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Sergey Alexandrovich Kryukov · Answer 1 · 2012-11-27T06:52:00

Solution 1

If you use P/Invoke, you can lean passing structures from here: http://msdn.microsoft.com/en-us/library/ef4c3t39%28v=vs.110%29.aspx[^].

Please see:
http://en.wikipedia.org/wiki/PInvoke[^],
http://msdn.microsoft.com/library/en-us/vcmxspec/html/vcmg_PlatformInvocationServices.asp[^].

This CodeProject article can also be useful: Essential P/Invoke[^].

But you can also avoid P/Invoke by using C++/CLI mixed-mode projects (managed + unmanaged). Please see:
http://en.wikipedia.org/wiki/C%2B%2B/CLI[^],
http://msdn.microsoft.com/en-us/library/xey702bw.aspx[^],
http://www.gotw.ca/publications/C++CLIRationale.pdf[^],
http://www.ecma-international.org/publications/standards/Ecma-372.htm[^].

—SA

Posted 27-Nov-12 6:52am

Sergey Alexandrovich Kryukov

Comments

Thomas Daniels 6-Dec-12 11:52am

Comment from the OP, posted as a non-answer (Solution 2):
Thanks for all that info Sergey.

Well, COM is needed because this is an (already released) device driver. The device is controlled, together with other devices, by a 3rd party software that uses COM for this purpose.

So it's something like this:

foobar.h


VARIANT_BOOL Connect(...);
VARIANT_BOOL Disconnect(void);
VARIANT_BOOL DoSomething(BYTE data1, BYTE data2, BYTE data3, BYTE data4);
VARIANT_BOOL DoSomethingToo(MyStruct* data);

foobar.cpp


BEGIN_DISPATCH_MAP(foobar, COleControl)
    DISP_FUNCTION_ID(foobar, "DoSomething", dispid_DoSomething, DoSomething, VT_BOOL, VTS_UI1 VTS_UI1 VTS_UI1 VTS_UI1)
    DISP_FUNCTION_ID(foobar, "DoSomethingToo", dispid_DoSomethingToo, DoSomethingToo, VT_BOOL, VTS_UI4)
END_DISPATCH_MAP()
 
VARIANT_BOOL foobar::DoSomethingToo(MyStruct* data)
{...}

foobar.idl


[uuid(...), ...]
library foobarLib
{
    ...
    #include "Structs.h"

    [uuid(...), ...]
    dispinterface _foobarInterface
    {
        methods:
            [id(3), helpstring("...")] VARIANT_BOOL DoSomethingToo(MyStruct* data);
    }
}

Structs.h



[uuid(...)]
typedef struct
{
	 unsigned char b[4];
} MyStruct;

It's all about DoSomethingToo() which is an attempt to replace DoSomething(). The problem with this code is, when called from C#, that there's an unhandled COMException TYPE_E_ELEMENTNOTFOUND. So i'm guessing there's something wrong (or missing) with the struct declaration.

crkn · Answer 2 · 2012-12-06T05:47:00

Thanks for all that info Sergey.

Well, COM is needed because this is an (already released) device driver. The device is controlled, together with other devices, by a 3rd party software that uses COM for this purpose.

So it's something like this:

foobar.h

C++

VARIANT_BOOL Connect(...);
VARIANT_BOOL Disconnect(void);
VARIANT_BOOL DoSomething(BYTE data1, BYTE data2, BYTE data3, BYTE data4);
VARIANT_BOOL DoSomethingToo(MyStruct* data);