Question

We have a base class: Filter. DistrFilter and ReportFilter inherit from Filter.

In another class, FilterService.cs, we have two functions that accept these three class types. FilterService operates on Filter objects but it does not inherit from anything.

public class FilterService
{

  public string GetDesc(List<T> filters) where T : Filter
  {
     if(filters.Count == 0) return String.Empty;

     StringBuilder s = new StringBuilder("<ul>");
     foreach (T f in filters)
         s.AppendFormat("<li>{1}</li>", GetFilterText(f));

     s.Append("</ul>");

     return s.ToString();
  }

  public string GetFilterText(Filter f)
  {
     return "filter";
  }

  public string GetFilterText(DistrFilter f)
  {
     return "distr filter";
  }

  public string GetFilterText(ReportFilter f)
  {
     return "report filter";
  }
}

public static void main(string[] args)
{
   List<DistrFilter> distrFilters = new List<DistrFilters>();
   distrFilters.Add(new DistrFilter());
   distrFilters.Add(new DistrFilter());
   distrFilters.Add(new DistrFilter());

   FilterService fs = new FilterService();
   Console.WriteLine(fs.GetDescription(distrFilters));
}

Then it prints:

• filter
• filter

…

How do I get it to print this instead?

• distr filter
• distr filter

…

Answer 1

Either implement GetFilterText() as a virtual method in Filter:

class Filter
{
    // Can be converted into a property as well.
    public virtual string GetFilterText { return "filter"; }
}

class DistrFilter : Filter
{
    public override string GetFilterText { return "distr filter"; }
}

Then do this:

StringBuilder s = new StringBuilder("<ul>");
foreach (T f in filters)
    s.AppendFormat("<li>{0}</li>", f.GetFilterText());

Or if you want to separate the concern of specifying the filter text from the Filter classes, use double dispatch (visitor pattern). This is useful if you can have different types of filter services. This can be done this way:

interface IServiceAcceptor
{
    string Accept(FilterService service);
}

public class Filter : IServiceAcceptor
{
    string IServiceAcceptor.Accept(FilterService service)
    {
        return service.GetFilterText(this);
    }   
}

public class DistrFilter : Filter, IServiceAcceptor
{
    string IServiceAcceptor.Accept(FilterService service)
    {
        return service.GetFilterText(this);
    }       
}

public class ReportFilter : Filter, IServiceAcceptor
{
    string IServiceAcceptor.Accept(FilterService service)
    {
        return service.GetFilterText(this);
    }       
}

Then in your service:

  public string GetDesc<T>(List<T> filters) where T : IServiceAcceptor
  {
     if(filters.Count == 0) return String.Empty;

     var s = new StringBuilder("<ul>");
     foreach (T f in filters)
         s.AppendFormat("<li>{0}</li>", f.Accept(this));

     s.Append("</ul>");

     return s.ToString();
  }

Some references: Double dispatch, Visitor Pattern