Extension interface patterns

Extension methods should be used as just that: extensions. Any crucial structure/design related code or non-trivial operation should be put in an object that is composed into/inherited from a class or interface.

Once another object tries to use the extended one, they won't see the extensions and might have to reimplement/re-reference them again.

The traditional wisdom is that Extension methods should only be used for:

• utility classes, as Vaibhav mentioned
• extending sealed 3rd party APIs

I think the judicious use of extension methods put interfaces on a more equatable position with (abstract) base classes.

Versioning. One advantage base classes have over interfaces is that you can easily add new virtual members in a later version, whereas adding members to an interface will break implementers built against the old version of the library. Instead, a new version of the interface with the new members needs to be created, and the library will have to work around or limit access to legacy objects only implementing the original interface.

As a concrete example, the first version of a library might define an interface like so:

public interface INode {
  INode Root { get; }
  List<INode> GetChildren( );
}

Once the library has released, we cannot modify the interface without breaking current users. Instead, in the next release we would need to define a new interface to add additional functionalty:

public interface IChildNode : INode {
  INode Parent { get; }
}

However, only users of the new library will be able to implement the new interface. In order to work with legacy code, we need to adapt the old implementation, which an extension method can handle nicely:

public static class NodeExtensions {
  public INode GetParent( this INode node ) {
    // If the node implements the new interface, call it directly.
    var childNode = node as IChildNode;
    if( !object.ReferenceEquals( childNode, null ) )
      return childNode.Parent;

    // Otherwise, fall back on a default implementation.
    return FindParent( node, node.Root );
  }
}

Now all users of the new library can treat both legacy and modern implementations identically.

Overloads. Another area where extension methods can be useful is in providing overloads for interface methods. You might have a method with several parameters to control its action, of which only the first one or two are important in the 90% case. Since C# does not allow setting default values for parameters, users either have to call the fully parameterized method every time, or every implementation must implement the trivial overloads for the core method.

Instead extension methods can be used to provide the trivial overload implementations:

public interface ILongMethod {
  public bool LongMethod( string s, double d, int i, object o, ... );
}

...
public static LongMethodExtensions {
  public bool LongMethod( this ILongMethod lm, string s, double d ) {
    lm.LongMethod( s, d, 0, null );
  }
  ...
}

Please note that both of these cases are written in terms of the operations provided by the interfaces, and involve trivial or well-known default implementations. That said, you can only inherit from a class once, and the targeted use of extension methods can provide a valuable way to deal with some of the niceties provided by base classes that interfaces lack :)

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Edit: A related post by Joe Duffy: Extension methods as default interface method implementations