Microsoft COM in c#

Question

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.