Restricting Interface Implementation

Under some circumstances, you may want to restrict users of an API to implement an interface. You may want to allow them to consume the interface but not to implement it in their own classes, so that, later, you can add new members to this interface without breaking the user's code. If retaining the interface as a public artifact is required, the programming language does not give you any option to enforce the desired restriction. Enter the InternalImplementAttribute from PostSharp.

Adding the constraint to the interface

To restrict implementation of publicly declared interfaces you simply need to add [InternalImplementAttribute] to that interface.

1. Place the caret over the interface that you want to add the attribute select the "Add architectural constraint...".

   

2. Select "Prevent interface implementation in a different assembly" and select Next.

   

3. Verify that you will be adding the InternalImplementAttribute attribute to the correct piece of code.

   

4. Once the download, installation and configuration of PostSharp have finished you can close the wizard and look at the changes that were made to your codebase.

   

5. You'll notice that the only thing that has changed in the code is the addition of the [InternalImplementAttribute] attribute.

   [InternalImplement] 
   public interface ICustomerRepository 
   { 
       IEnumerable<Customer> FetchAll(); 
   }
   

Once that is done, implementing the interface that was decorated with the InternalImplementAttribute from another assembly will create a compile time warning.

Emitting an error instead of a warning

If a warning isn't strong enough for your environment you can change the output to a compile time error by setting the InternalImplementAttribute to have a Severity type of Error.

[InternalImplement(Severity = SeverityType.Error)] 
public interface ICustomerRepository 
{ 
    IEnumerable<Customer> FetchAll(); 
}

Now any reference to the decorated interface from another assembly will generate an error and fail the compilation of your project.

Ignoring warnings

If you are trying to implement a constrained interface in a separate assembly and you want to override the warning being generated there is a solution available for you. The SuppressWarningAttribute attribute can be applied to stop warnings from being generated.

To suppress warnings all that you need to do is add the SuppressWarningAttribute attribute to the offending piece of code. In this example, we would suppress the warning being generated by adding the attribute to the class that is implementing the constrained interface. Once we have done that, the warning generated for that specific implementation would be suppressed. All other locations that are implementing this interface will continue to generate their warnings.

[IgnoreWarning("AR0101")] 
public class PreferredCustomerRepository : ICustomerRepository 
{ 
    public IEnumerable<Customer> FetchAll() 
    { 
        return null; 
    } 
}

See Also

Reference

InternalImplementAttribute
SuppressWarningAttribute
Other Resources

Controlling Component Visibility Beyond Private and Internal