how to write test case using NUnit which has a dependency of intefaces? : public List<CustomerViewModel> GetResellerCustomersWithProperties(string shortCode)
        {            
            var businessManager = 
            DependencyContainer.GetInstance<ICortexBusinessManager>();
            return businessManager.GetResellerCustomersWithProperties(shortCode); 
        }

How do we write Test case using Nunit which has a dependency from interfaces.