Abstract class does not implement interface

Make sure methods in the base class have the same name as the interface, and they are public. Also, make them virtual so that subclasses can override them without hiding them.

interface IInterface {
   void Do();
   void Go();
}

abstract class ClassBase : IInterface {

    public virtual void Do() {
         // Default behaviour
    }

    public abstract void Go();  // No default behaviour

}

class ConcreteClass : ClassBase {

    public override void Do() {
         // Specialised behaviour
    }

    public override void Go() {
        // ...
    }

}

Move the interface methods into the abstract class and declare them abstract as well. By this, deriving classes are forced to implement them. If you want default behaviour, use abstract classes, if you want to only have the signature fixed, use an interface. Both concepts don't mix.