IComparable CompareTo(), How can I implement comparison on 2 different fields

Question

I Have a class like so :

public class Incident : IComparable<Incident>
{
    public int Id { get; set; }
    public string Description { get; set; }
    public string IncidentType { get; set; }

    public int CompareTo(Incident other)
    {
        string str = other.Description;
        int ret = -1;
        if (String.Compare(Description, str, StringComparison.OrdinalIgnoreCase) < 0)
            ret = 1;
        else if (String.Compare(Description, str, StringComparison.OrdinalIgnoreCase) > 0)
            ret = -1;
        else if (String.Compare(Description, str, StringComparison.OrdinalIgnoreCase) == 0)
            ret = 0;

        return ret;
    }
}

I can sort objects of this class alphabetically based on the description field. How can I sort the class based on IncidentType field ?

I do not want to sort them on both fields simultaneously. Sometimes I want to sort incidents by Description, sometimes on IncidentType

Answer 1

If it's a typical case for you to sort either by Description or by IncidentType, you can implement different Comparers to sort on different conditions

public class Incident: IComparable<Incident> {
  ...

  private IncidentTypeComparerClass: IComparer<Incident> {
    public int Compare(Incident x, Incident y) {
      if (Object.ReferenceEquals(x, y))
        return 0;
      else if (Object.ReferenceEquals(x, null))
        return -1;
      else if (Object.ReferenceEquals(null, y))
        return 1;

      return String.Compare(x.IncidentType, y.IncidentType, StringComparison.OrdinalIgnoreCase);
    }    
  }

  // Additional comparer, by IncidentType 
  public static readonly ByIncidentTypeComparer: IComparer<Incident> = new IncidentTypeComparerClass();

  ...
}

...

List<Incident> incidents = ...

// Sort by IncidentType    
incidents.Sort(Incident.ByIncidentTypeComparer);
...
// Default sort (that's by Description)
incidents.Sort();

In case you want to sort by IncidentType only once or twice you can do it explicitly:

List<Incident> incidents = ...

incidents.Sort((x, y) => String.Compare(x.IncidentType, 
                                        y.IncidentType, 
                                        StringComparison.OrdinalIgnoreCase));

Answer 2

You can implement IComparer<T> in seperate class and you can use this comparer when you sorting your elements:

public class IncidentComparer : IComparer<Incident>
{
    public int Compare(Incident x, Incident y)
    {
        return x.IncidentType.CompareTo(y.IncidentType);
    }
}

For example if you have a list like this, you can use your comparer:

List<Incident> incidents = new List<Incident>();
...
incidents.Sort(new IncidentComparer());

Or alternatively you can use LINQ instead:

incidents = incidents.OrderBy(x => x.IncidentType).ToList();