lines.cs

interface ILine
{}

class AdditionLine : ILine {}
class SubtractionLine : ILine {}
class ModificationLine : ILine {}

void ProcessLines(IList<ILine> lines) {}

var additions = new List<AdditionLine>();

ProcessLines(additions) // blegh, doesn't work

// another...

interface ISnippet
{
  IEnumerable<ILine> Lines { get; }
}

class AdditionSnippet
{
  private List<AdditionLine> lines = new List<AdditionLine>();

  // Blegh, fails. Gotta use lines.Cast<ILine>();
  public IEnumerable<ILine> Lines
  {
    get { return lines; }
  }

  public AdditionSnippet(IEnumerable<AdditionLine> additions)
  {
    lines = new List<AdditionLine>(additions);
  }
}