How can I activate an open MS Project File (MPP)

I am using Microsoft.Office.Interop.MSProject to work with MPP files. My requirement is to open mutliple mpp files in Application objects and update them as required. However, The last opened mpp file is always set as ActiveProject. Due to which all updations are made to last MPP file.

I tried using Project.Activate() but I get exception.How can I activate desired project (MPP) on demand by keeping multiple files open in 'Application' object?

I am using VS2010, .NET Framework 4.0, Microsoft Project 14.0 Object Library.