how to define this?

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ZhangZQ - 21 Jul 2004 08:53 GMT

if there is a C function in a DLL(My.dll) has this declaration,

struct MyStruct
{
int a;
int b;
};

int MyFunc(int param1, MyStruct** param2);

how to define this in C# ? if there is another C function like this int
MyFunc1(int param1, char** szRtn);, I can define it as

[DllImport("My.Dll")]
int MyFunc1(int param1, out string szRtn);

I want to know how to define the MyFunc?

Thanks!

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Fabian Schmied - 22 Jul 2004 07:51 GMT

> if there is a C function in a DLL(My.dll) has this declaration,
>
[quoted text clipped - 7 lines]
>
> how to define this in C# ?

In C#, don't define MyStruct as a struct, define it as a class. This will
add an additional layer of indirection. Then,

[DllImport("My.dll")]
static extern int MyFunc(int param1, ref MyStruct param2);

should work, I think.

(If this is really a C function, you might need to use
CallingConvention.Cdecl the default is StdCall.)

Fabian

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ZhangZQ - 22 Jul 2004 08:37 GMT

Thanks for your reply, but it doesn't work, I got this message, "Unhandled
Exception: System.InvalidCastException: Cannot cast from source type to
destination type."

I want to know the different definition of these 2 C functions in C#, int
MyFunc(int param1, MyStruct** param2); and int MyFunc(int param1, MyStruct*
param2);, what is the different declaration in C#?

Regards,
ZhangZQ

> > if there is a C function in a DLL(My.dll) has this declaration,
> >
[quoted text clipped - 20 lines]
>
> Fabian

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Fabian Schmied - 22 Jul 2004 09:48 GMT

> Thanks for your reply, but it doesn't work, I got this message,
> "Unhandled
> Exception: System.InvalidCastException: Cannot cast from source type to
> destination type."

Hmm, I just tested it with a DLL I quickly wrote, and it seems to work
alright. Where exactly do you get this exception?

> I want to know the different definition of these 2 C functions in C#, int
> MyFunc(int param1, MyStruct** param2); and int MyFunc(int param1,
> MyStruct*
> param2);, what is the different declaration in C#?

int MyFunc(int param1, MyStruct** param2);
becomes:
(with MyStruct being a C# class)
static extern int MyFunc(int param1, ref MyStruct param2);
(I don't know of a way to do this with MyStruct being a C# struct)

int MyFunc(int param1, MyStruct* param2);
becomes:
(with MyStruct being a C# class)
static extern int MyFunc(int param1, MyStruct param2);
(with MyStruct being a C# struct)
static extern int MyFunc(int param1, ref MyStruct param2);

So, you see, "class" adds one indirection (*), "ref" also does.

Of course, if you don't need to use param2 in your C# program or if you
only pass it to another unmanaged function as an opaque handle, you can
also use IntPtr or ref IntPtr.

Fabian

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ZhangZQ - 22 Jul 2004 10:12 GMT

Thank you very much!

It works now, I am using ref IntPtr, that param2 is only to be passed to the
unmanaged code.

Regards,
ZhangZQ

> > Thanks for your reply, but it doesn't work, I got this message,
> > "Unhandled
[quoted text clipped - 29 lines]
>
> Fabian

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how to define this?

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