Hi, I notice that Marshal.GetNativeVaraintForObject is a way to create
a variant from object.

But DotNet can do marshalling from object to variant when an object is
used as a parameter.

Can anyone tell in which case I should use
Marshal.GetNativeVaraintForObject ?

You need it if you do custom marshalling.

I used it when I created a com-wrapper in c++/cli.

Unfortunatly there is no corresponding FreeNativeVariant so you will have
memory leaks unless you call some native function. (I think it is called
VariantClear in ole.dll)