Calling method of another dll of interface

Question

In my code , extern alias OnlineQBO is my alias of DevDefined.OAuth.dll, i have second dll is XeroApi.dll
whenever my following line of code is executes , conReq object creating by the DevDefined.OAuth.dll , but object call the Get() method of XeroApi.dll , so error ocuring as An explicit conversion exists(are you missing a cast?)

OnlineQBO::DevDefined.OAuth.Consumer.IConsumerRequest conReq = oSession.Request();
 conReq = conReq.Get();

Can you suggest any anwser please?