[Solution] The type 'System.IO.Packaging.Package' is defined in an assembly that is not referenced

Here Mudassar Ahmed Khan has explained how to solve the following compilation error: CS0012: The type 'System.IO.Packaging.Package' is defined in an assembly that is not referenced. You must add a reference to assembly 'WindowsBase, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. The above error occurs when we use the OpenXml Packaging Library.