My attempt at Roy Osherove's TDD Kata 1 ended up with this skeleton

gistfile1.cs

public class Math
{
 
 private _parser As IParser;
 
 public Math(IParser parser)
 {
   this._parser = parser;
 }
 
 
 public int Add(string numbers)
 {
   IEnumerable<int> numbers = this._parser.ParseNumbers(numbers)
   //Add them up and return the value
 }
  
}

public class Parser : IParser
{
  public IEnumerable<int> ParseNumbers(string numbers)
  {
    //Let's assume I do this correctly here
  }
}

public interface IParser 
{
  IEnumerable<int> ParseNumbers(string numbers);
}