Question

I have a specialized list that holds items of type IThing:

public class ThingList : IList<IThing> {...}  public interface IThing {     Decimal Weight { get; set; }     Decimal Velocity { get; set; }     Decimal Distance { get; set; }     Decimal Age { get; set; }     Decimal AnotherValue { get; set; }      [...even more properties and methods...] } 

Sometimes I need to know the maximum or minimum of a certain property of all the things in the list. Because of ‘Tell don’t ask’ we let the List figure it out:

public class ThingList : IList<IThing> {     public Decimal GetMaximumWeight()     {         Decimal result = 0;         foreach (IThing thing in this) {             result = Math.Max(result, thing.Weight);         }         return result;     } } 

Thats very nice. But sometimes I need the minimum weight, sometimes the maximum velocity and so on. I don’t want a GetMaximum*()/GetMinimum*() pair for every single property.

One solution would be reflection. Something like (hold your nose, strong code smell!):

Decimal GetMaximum(String propertyName); Decimal GetMinimum(String propertyName); 

Are there any better, less smelly ways to accomplish this?

Thanks, Eric

Edit: @Matt: .Net 2.0

Conclusion: There is no better way for .Net 2.0 (with Visual Studio 2005). Maybe we should move to .Net 3.5 and Visual Studio 2008 sometime soon. Thanks, guys.

Conclusion: There are diffent ways that are far better than reflection. Depending on runtime and C# version. Have a look at Jon Skeets answer for the differences. All answers are are very helpful.

I will go for Sklivvz suggestion (anonymous methods). There are several code snippets from other people (Konrad Rudolph, Matt Hamilton and Coincoin) which implement Sklivvz idea. I can only ‘accept’ one answer, unfortunately.

Thank you very much. You can all feel ‘accepted’, altough only Sklivvz gets the credits 😉

Answer 1

Yes, you should use a delegate and anonymous methods.

For an example see here.

Basically you need to implement something similar to the Find method of Lists.

Here is a sample implementation

public class Thing {     public int theInt;     public char theChar;     public DateTime theDateTime;          public Thing(int theInt, char theChar, DateTime theDateTime)     {         this.theInt = theInt;         this.theChar = theChar;         this.theDateTime = theDateTime;     }          public string Dump()     {         return string.Format("I: {0}, S: {1}, D: {2}",              theInt, theChar, theDateTime);     } }  public class ThingCollection: List<Thing> {     public delegate Thing AggregateFunction(Thing Best,                          Thing Candidate);          public Thing Aggregate(Thing Seed, AggregateFunction Func)     {         Thing res = Seed;         foreach (Thing t in this)          {             res = Func(res, t);         }         return res;     } }  class MainClass {     public static void Main(string[] args)     {         Thing a = new Thing(1,'z',DateTime.Now);         Thing b = new Thing(2,'y',DateTime.Now.AddDays(1));         Thing c = new Thing(3,'x',DateTime.Now.AddDays(-1));         Thing d = new Thing(4,'w',DateTime.Now.AddDays(2));         Thing e = new Thing(5,'v',DateTime.Now.AddDays(-2));                  ThingCollection tc = new ThingCollection();                  tc.AddRange(new Thing[]{a,b,c,d,e});                  Thing result;          //Max by date         result = tc.Aggregate(tc[0],              delegate (Thing Best, Thing Candidate)              {                  return (Candidate.theDateTime.CompareTo(                     Best.theDateTime) > 0) ?                      Candidate :                      Best;               }         );         Console.WriteLine("Max by date: {0}", result.Dump());                  //Min by char         result = tc.Aggregate(tc[0],              delegate (Thing Best, Thing Candidate)              {                  return (Candidate.theChar < Best.theChar) ?                      Candidate :                      Best;              }         );         Console.WriteLine("Min by char: {0}", result.Dump());                    } } 

The results:

Max by date: I: 4, S: w, D: 10/3/2008 12:44:07 AM
Min by char: I: 5, S: v, D: 9/29/2008 12:44:07 AM