Question

I have two lists of custom objects and want to update a field for all objects in one list if there is an object in the other list which matches on another pair of fields.

This code explains the problem better and produces the results I want. However for larger lists 20k, and a 20k list with matching objects, this takes a considerable time (31s). I can improve this with ~50% by using the generic lists Find(Predicate) method.

using System; using System.Linq; using System.Linq.Expressions; using System.Collections.Generic; namespace ExperimentFW3 {     public class PropValue     {         public string Name;         public decimal Val;         public decimal Total;     }     public class Adjustment     {         public string PropName;         public decimal AdjVal;     }     class Program     {         static List<PropValue> propList;         static List<Adjustment> adjList;          public static void Main()         {             propList = new List<PropValue>{                 new PropValue{Name = 'Alfa', Val=2.1M},                 new PropValue{Name = 'Beta', Val=1.0M},                 new PropValue{Name = 'Gamma', Val=8.0M}             };             adjList = new List<Adjustment>{                 new Adjustment{PropName = 'Alfa', AdjVal=-0.1M},                 new Adjustment{PropName = 'Beta', AdjVal=3M}             };              foreach (var p in propList)             {                 Adjustment a = adjList.SingleOrDefault(                     av => av.PropName.Equals(p.Name)                     );                 if (a != null)                     p.Total = p.Val + a.AdjVal;                 else                     p.Total = p.Val;             }         }     } } 

The desired result is: Alfa total=2,Beta total=4,Gamma total=8

But I wonder if this is possible to do even faster. Inner joining the two lists takes very little time, even when looping over 20k items in the resultset.

var joined = from p in propList              join a in adjList on p.Name equals a.PropName              select new { p.Name, p.Val, p.Total, a.AdjVal }; 

So my question is if it’s possible to do something like I would do with T-SQL? An UPDATE from a left join using ISNULL(val,0) on the adjustment value.

Answer 1

That join should be fairly fast, as it will first loop through all of adjList to create a lookup, then for each element in propList it will just use the lookup. This is faster than your O(N * M) method in the larger code – although that could easily be fixed by calling ToLookup (or ToDictionary as you only need one value) on adjList before the loop.

EDIT: Here’s the modified code using ToDictionary. Untested, mind you…

var adjDictionary = adjList.ToDictionary(av => av.PropName); foreach (var p in propList) {     Adjustment a;     if (adjDictionary.TryGetValue(p.Name, out a))     {         p.Total = p.Val + a.AdjVal;     }     else     {         p.Total = p.Val;     } }