gistfile1.cs

public class LessThanOrEqualFilterRule : IFilterRule
{
  public IEnumerable<Widget> Filter(IEnumerable<Widget> widgets)
  {
    return widgets.Where(w => w.foo <= 100);
  }
}

public class EqualFilterRule : IFilterRule
{
  public IEnumerable<Widget> Filter(IEnumerable<Widget> widgets)
  {
    return widgets.Where(w => w.bar == <no idea what baz is>);
  }
}

public class FilterChooser
{
  public IFilterRule ChooseFilter(bool b)
  {
    return b ? lessThanOrEqualFilterRule : equalFilterRule; // populated via DI
  }
}

// ...

var filter = filterChooser.ChooseFilter(b);
var widgets = filter.Filter(q);