I am trying to manipulate xmp metadata of images. I can do it in c# or c++.
How can I do this? Is there any library that I can use?
A C# port of Adobe's XmpCore library for Java is available here:
https://github.com/drewnoakes/xmp-core-dotnet
If you know the Java/C++ API, it's basically identical except the API looks like a .NET API, with .NET conventions.
For c++, you can use the latest XMP SDK. Have a look at http://www.adobe.com/devnet/xmp.html for instructions to get latest source and documentation for building and using it.