Restricting Interface Implementation

You may want to prevent users of an API from implementing an interface. They can consume the interface but not implement it in their own classes, so that you can later add new members to this interface without breaking the code of the users. If the interface must remain public, the programming language has no option to enforce this restriction. The InternalImplementAttribute attribute of PostSharp provides this restriction.

Adding the constraint to the interface

To restrict the implementation of a publicly declared interface, add [InternalImplementAttribute] to that interface.

1. Place the caret on the interface to which you want to add the attribute, and select "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. When the download, installation and configuration of PostSharp have finished, close the wizard and look at the changes that were made to your codebase.

   

5. Notice that the only change in the code is the addition of the [InternalImplementAttribute] attribute.

   Full screen

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

After that, implementing the interface that is decorated with InternalImplementAttribute from another assembly creates the build-time warning AR0101, unless the assembly of the interface grants access to that assembly with InternalsVisibleToAttribute.

Emitting an error instead of a warning

If a warning is not strict enough, you can change the output to a build-time error by setting the Severity property of InternalImplementAttribute to Error.

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

Now any implementation of the decorated interface in another assembly generates an error and fails the build of your project.

Ignoring warnings

If you implement a constrained interface in a separate assembly and you want to suppress the warning that is generated, apply the SuppressWarningAttribute attribute.

To suppress the warning, add the SuppressWarningAttribute attribute to the offending code. In this example, add the attribute to the class that implements the constrained interface. The warning for that specific implementation is then suppressed. All other locations that implement this interface still generate their warnings.

[SuppressWarning("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