IImporter.cs

public interface IImportEngine<TSource, TTarget> where TSource : ExternalSourceModel
                                                 where TTarget : IIdentityEntity
{
 Task Import();
}

Importer.cs

public class ImportEngine<TSource, TTarget> : IImportEngine<TSource, TTarget> where TSource : ExternalSourceModel
                                                                              where TTarget : IIdentityEntity
{
  private readonly IImporter<TSource, TTarget> _importer;
  public ImportEngine(IImporter<TSource, TTarget> importer)
  {
    _importer = importer;
  }
  public async Task Import()
  {
    var source = await _importer.GetExternalSource();
    foreach (var item in source)
    {
      if (_importer is IValidator<TSource>)
      {
        var validator = _importer as IValidator<TSource>;
        string validationMessage;
        if (!validator.Validate(item, out validationMessage))
        {
          // TODO: Log validation message.
          continue;
        }
      }
      var target = _importer.GetTarget(item);
      if (target == null)
      {
        if (_importer is IInserter<TSource>)
        {
          var inserter = _importer as IInserter<TSource>;
          inserter.Insert(item);
        }
      }
      else if (_importer is IUpdater<TSource>)
      {
        var updater = _importer as IUpdater<TSource>;
        updater.Update(item);
      }
    }
    // TODO: Commit changes.
  }
}