How to access a pointer to an existing structure?

Hi,
Iam trying to implement the QueryStatus method of IOleCommandTarget
interface.
public int QueryStatus(ref Guid pguidCmdGroup, int cCmds, IntPtr
prgCmds, IntPtr pCmdText)
{
object ocmd=Marshal.PtrToStructure(prgCmds,typeof(OLECMD));

}
Basically, prgCmds is a pointer to an existing OLECMD structure, and I
want to fill in that structure. I tried to use Marshal.PtrToStructure
to obtain the structure. But it returns an object.The structure
definition of OLECMD is as follows:
[StructLayout(LayoutKind.Sequential)]
public struct OLECMD
{
public UInt32 cmdID;
public UInt64 cmdf;
}
I want to set the value of cmdf. How to do it?Please somebody help me
in this regard.

Thanks in advance,
Sujoan.