Could not load file or assembly 'RevitAPI.dll' when I want to use FieldType of a type in Revit API

I have two class libraries core and plugins and a WPF application that uses these two libraries. In core I dynamically load the plugins as follows:

try
        {
            Assembly assembly = Assembly.LoadFile("plugins.dll");
        }

After I load the plugins.dll I get the types in the plugins that has implemented Node abstract class from core library, that is a class defined in the core. This is the scenario that I used to develop and extensible application. Somewhere in my core library I need to traverse all fields of Node classes loaded from plugins. It works nice for all fields like int, double and other custom classes that are defined inside plugins library.

theList = assembly.GetTypes().ToList().Where(t => t.BaseType == typeof(Node)).ToList();
var fieldInfos = theList[0].GetType().GetRuntimeFields();
foreach (var item in fieldInfos)
        {
            Type type = item.FieldType; 
            // Here I get exception for fields like XYZ that defined in
            // Revit API though for fields like Int and double it works charm
        }

But the problem is that in plugins project, I also use Revit API, and when the above loop reaches to a field that comes from RevitAPI.dll I get the following exception (I tried target platform Any and x86):

An unhandled exception of type 'System.BadImageFormatException' occurred in mscorlib.dll
Additional information: Could not load file or assembly 'RevitAPI, 
Version=2015.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its 
dependencies. An attempt was made to load a program with an incorrect format.

When I change the target platform in build section of all 3 projects to x64 I get this exception, instead:

An unhandled exception of type 'System.IO.FileNotFoundException' occurred in mscorlib.dll
Additional information: Could not load file or assembly 'RevitAPI.dll' 
or one of its dependencies. The specified module could not be found.