What types should I use in this COM Interface

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Oolis Kraprin - 29 Feb 2008 08:48 GMT

Hi, I am having to write a COM wrapper dll to enable the use of a c++
library in a c# .net app, and have a few questions regarding what
types to use in the interface.

I used to program quite a bit of COM, but a couple of years of c# .net
has made me stupid.

Anyway, I need to pass (from c# to the com component) a HWND window
handle, and 2 strings. The function needs to deliver back to c# 2
lists or arrays of strings.

My guess at the moment is something like:
ULONG WindowHandle
BSTR FirstString
BSTR SecondString
VARIANT* FirstReturnedStringArray or List
VARIANT* SecondReturnedStringArray or List

and I probably need to return 2 ints indicating the length of the 2
lists right, or is there some type for representing a list of array of
strings that contains the count?

I am looking for whatever makes the most sense and requires the
minimal amount of mucking aroung with marshalling etc.

Thanks a lot

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Jeroen Mostert - 29 Feb 2008 18:50 GMT

> Hi, I am having to write a COM wrapper dll to enable the use of a c++
> library in a c# .net app, and have a few questions regarding what
[quoted text clipped - 9 lines]
> My guess at the moment is something like:
> ULONG WindowHandle

There are plenty of ways to marshal an HWND, but the easiest and still
bit-safe way is to use a HANDLE as the native type (and cast between HWND
and HANDLE on the C++ end).

> BSTR FirstString
> BSTR SecondString
[quoted text clipped - 4 lines]
> lists right, or is there some type for representing a list of array of
> strings that contains the count?

SAFEARRAY. The IDL could look like this:

[uuid(...)]
library Test {
[object, uuid(...)]
interface ITest : IUnknown {
HRESULT Foo([in] HANDLE hwnd, [in] BSTR firstString, [in] BSTR
secondString, [out] SAFEARRAY(BSTR)* firstArray, [out] SAFEARRAY(BSTR)*
secondArray);
}
}

Put this in a type library and let tlbimp.exe do its magic, and it should
produce something that looks a lot like this without the need for any
marshalling:

void Foo(IntPtr hwnd, string firstString, string secondString, out string[]
firstArray, out string[] secondArray);

Using SAFEARRAYs in C++ is a bit of a chore, but the ATL helper clas
CComSafeArray goes a long way towards easing the pain.

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Oolis Kraprin - 03 Mar 2008 07:47 GMT

Thanks a lot for the reply.

I ended up with something like this:
HRESULT Address([in] ULONG windowHandle, [in] BSTR firstString, [in]
BSTR secondString, [out] VARIANT *firstArray, [out] VARIANT
*secondArray);

The ulong seems to work ok for the window handle, but I might change
it to HANDLE. I managed to specify SAFEARRAYs in the IDL but had
problems with using them in the c++ file function definition. I ended
using safearrays by wrapping them in the variants.

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What types should I use in this COM Interface

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