Microsoft COM in c#

manju M

Hi
Iam new to this COM.
I need to convert a string to VT_BSTR.
One method is I have my string in c# and i need to pass this as parameter in c++ application where i need to convert it into VT_bstr and return the result in c#.How to do this and where i wil hold the VT_bstr variable in c#.
Another method is how to convert string to VT_bstr in c# itself.
Please help me on this.

5 Replies

Know the answer? Post it — somebody with the same question will find it here.

Sam Hobbs16y ago
Accepted answer
VT_BSTR is not a type; it is a code that indicates that a variant object has a bstr in it. A bstr is always created by Windows, but I think that all striings in C# are like bstrs.

So you need to check the documentation for use of the variant type in C#; that is, how to tell C# that a parameter is a variant.

If the C# truly is a COM object, then you don't need to be concerned about details; just add a reference to the COM object and let C# take care of details. Are you sure it is a COM object?

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Achal Kumar10y ago
When a string is sent from C# to COM . RCW will automatically convert it into BSTR string.
You will always accept a C# string in COM as bstr. It doesn't need explicit conversion.

Regards,
Achal
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Zbynek Zahradnik12y ago
You may want save yourself a lot of trouble by using an OPC client toolkit instead, something like : http://www.opclabs.com/products/quickopc/languages-and-tools/csharp
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Sam Hobbs16y ago
I am sorry but if you want more details you need to provide relevant details. What does the C++ function look like in terms of what are it's parameters? Also, what type of interface does it use? Is it a COM object with a type library or is it a DLL without a type library?
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manju M16y ago
Hi
Thanks for your reply.
My requirement is i need to fetch value from database and to write the value in AIM OPC server.
The OPC server supports variant type only.
I need to convert the value which is in string in database to variant.
when i referred some dcoument for AIM OPC server, it s given that the equivalent of string is vt_bstr. so i need to convert this into that.
Wat reference i need to add or how to do this?
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