How to Resolve 'An entity object cannot be referenced by multiple instances of IEntityChangeTracker' in Entity Framework 4.1?

Troubleshooting Entity Object Reference Conflicts in Entity Framework 4.1

Saving an Employee entity linked to a City entity in Entity Framework 4.1 can sometimes throw the exception: "An entity object cannot be referenced by multiple instances of IEntityChangeTracker." This usually happens when adding related entities.

The problem stems from how the EmployeeService and CityService classes manage their contexts. The line payrollDAO.AddToEmployee(e1); in EmployeeService is the culprit. Here, e1 (the Employee entity) is added to a context already holding a reference to city1 (the linked City entity). This creates a conflict because an entity can't be attached to multiple contexts at once.

The solution is to ensure both services share the same context. This can be done in two ways:

1. Dependency Injection: Inject the context into the service constructors:

<code class="language-csharp">EmployeeService es = new EmployeeService(context);
CityService cs = new CityService(context);</code>

This ensures both services operate within the same context, preventing conflicts.

2. Consolidated Service: Create a single service, for example, EmployeeCityService, to manage both Employee and City entities. This single service would use a single context, eliminating the possibility of context clashes. This approach promotes better data integrity and simplifies context management.

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