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I'm hosting a .NET library in my C++ program using the following methods, though not an exhaustive list:

CorBindToRuntimeEx()
GetDefaultDomain()
CreateInstance()
GetIDsOfNames()

And eventually a call to Invoke().

This works fine for basic types, enums, strings, and arrays. However, I cannot figure out how to pass a struct. Here's a skeleton of what I have now:

//library.cs  
public class AStruct
{
    public int i1;
    public string s1;
    public double d1;
}  

//...
public AStruct getAStruct();

//interop.cpp  
HRESULT hr = assembly->Invoke (id_getAStruct, ...);

The OUT PARAM return value of this function is a VARIANT with type VT_DISPATCH.

When I view retVal.pdispVal in my debugger, I can see that the contents of my struct are not near that address. I'd like to use varIDis.pdispVal->QueryInterface() to access my struct, but I have no idea what the IID is, nor how to discover it.

Also, I don't have the source code to the .NET library, though I can see much of it with Reflector. I'm using a test library I wrote in .NET to figure out how to proceed.

So, how can I pass and receive structs between .NET and C++ using COM?

Thanks greatly.

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4
  • Is there any way you can get information about this .NET library? It would make it easier. Surely there is some documentation. I presume you know how to define the struct and its marshalling requirements. The issue is getting the struct out of the VARIANT. Commented Mar 7, 2011 at 21:17
  • Yes, the .NET library is not obfuscated so I can see what they're doing in the first layer of code. However the documentation appears to be autogenerated and contains far less information than I can see in Reflector. I believe I've declared my C++ struct correctly, not that it matters until I can convert my VARIANT. Commented Mar 7, 2011 at 21:30
  • You really need to talk to the programmer that wrote this code. You're not very close. Hosting the CLR yourself and using COM to call managed code isn't untrivial. For the record, you'll need IDispatch to retrieve the object properties. Only late binding is supported here. Not great. Commented Mar 8, 2011 at 0:36
  • Hans, thanks for the response. To clarify, I wrote all of the C++ code. The author(s) of the .NET code expected everyone to use it from .NET, and I don't believe they know much about interop in this direction. Do you have a link to help? Google has not helped much with this question. Commented Mar 8, 2011 at 19:04

1 Answer 1

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I don't think your problem is related to structs. In fact, there are no structs involved here - not in C#, nor C++ nor COM. The getAStruct method (of some unspecified class?) returns a pointer to the IDispatch interface on an instance of the AStruct class.

You don't have enough hard information to do things properly, so this is how I'd approach it... (lots of guessing...)

Forget structs. You're stuck with an IDispatch*. If you know the full definition of the AStruct class, or at least the full list of its properties and the order they're defined in, then start with this assumption: the DISPID of the first property is 0x6002000. The next is 0x6002001, and so on. This isn't guaranteed, of course, but you might be lucky. And I forget the conditions that cause the DISPIDs to start at 0x60020000, so it might be totally wrong. Also, I don't suppose there's any guarantee that the properties' DISPIDs go in the same order as their definitions in the source code.

Moving breezily along, call IDispatch::Invoke on the IDispatch* you've got, passing the guessed DISPIDs and wFlags=DISPATCH_PROPERTYGET/PUT.

Like I said, lots of guessing. But if my back was against the wall, this is where I'd start.

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5 Comments

Alright, forgive my skepticism. Your method worked. Specifically, once I received the variant of type VT_DISPATCH as the "VarResult" from my Invoke() call, I called var.pdispVal->GetIDsOfNames() for each member name, and then called var.pdispVal->Invoke() to retrieve the value. I used 0 for IID, DISPATCH_PROPERTYGET, and empty params. Thanks! Unfortunately now I really wonder how I'm going to create a struct to pass into other functions.
@David That shouldn't be too hard since you now know once side of the interface!
@David: Ha! I was so focused on your problem description that I completely forgot about GetIDsOfNames. Glad you found the right way to do it. About those structs... Is it a genuine C# struct in the real .NET DLL? (As opposed to the test one you wrote.) A managed struct maps to a VARIANT of type VT_RECORD, containing a pointer to an IRecordInfo interface. If you're reading the struct on the COM side then the IRecordInfo* will be ready for you. If you're sending the struct from COM to .NET then you'll need to get your hands on an IRecordInfo* - check out GetRecordInfoFromTypeInfo.
@Ciaran: I'm not sure I know the answer, but I'll give the details I know. Each of these "structs" is a class with all public data members of simple types (int, float, string, int[], etc.). The only member functions they have are constructors. In most cases structs are passed into functions (as ref or not), but I started here because it gives me a variant to inspect. Thus far, I have not seen VT_RECORD.
In C# there's a big difference between structs and classes, unlike in C++ where the only difference is the default visibility of members. So if you've really got C# classes, then your IDispatch stuff is right and you can forget about VT_RECORD. To pass an instance of the managed class to a managed method is easy enough if you already have the instance - just pass its IDispatch* in a VARIANT. But if you have to create the instance first, then you'll need to call ClrCreateManagedInstance. (I haven't done this before.)

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