Compare two Lists for differences

Question

I would like some feedback on how we can best write a generic function that will enable two Lists to be compared  The Lists contain class objects and we would like to iterate through one list  looking for the same item in a second List and report any differences   We already have a method to compare classes  so we need feedback on how we can feed the method  shown below  from two Lists   For example  say we have a simple  Employee  class that has three properties  Name  ID  Department  We want to report the differences between List and another List   Note  Both lists will always contain the same number of items     As mentioned above  we have a generic method that we use to compare two classes  how can we incorporate this method to cater for Lists  i e  from another method  loop through the List and feed the classes to the generic method      but how do we find the equivalent class in the second List to pass to the method below   public static string CompareTwoClass ReturnDifferences lt T1  T2 gt  T1 Orig  T2 Dest      where T1   class     where T2   class          Instantiate if necessary     if  Dest    null  throw new ArgumentNullException  Dest    Destination class must first be instantiated          var Differences   CoreFormat StringNoCharacters          Loop through each property in the destination       foreach  var DestProp in Dest GetType   GetProperties                     Find the matching property in the Orig class and compare         foreach  var OrigProp in Orig GetType   GetProperties                           if  OrigProp Name    DestProp Name    OrigProp PropertyType    DestProp PropertyType  continue              if  OrigProp GetValue Orig  null  ToString      DestProp GetValue Dest  null  ToString                    Differences   Differences    CoreFormat StringNoCharacters                        string Format   0    1  - gt   2    OrigProp Name                                                         OrigProp GetValue Orig  null                                                          DestProp GetValue Dest  null                          string Format   0   1  2    3  - gt   4    Differences                                                                Environment NewLine                                                                OrigProp Name                                                                OrigProp GetValue Orig  null                                                                 DestProp GetValue Dest  null                        return Differences      Any suggestions or ideas appreciated   Edit  Targeting  NET 2 0 so LINQ is out of the question

User · Answer

This approach from Microsoft works very well and provides the option to compare one list to another and switch them to get the difference in each. If you are comparing classes simply add your objects to two separate lists and then run the comparison.

http://msdn.microsoft.com/en-us/library/bb397894.aspx

User · Answer

I think you re looking for a method like this   public static IEnumerable lt TResult gt  CompareSequences lt T1  T2  TResult gt  IEnumerable lt T1 gt  seq1      IEnumerable lt T2 gt  seq2  Func lt T1  T2  TResult gt  comparer        var enum1   seq1 GetEnumerator        var enum2   seq2 GetEnumerator         while  enum1 MoveNext    amp  amp  enum2 MoveNext                  yield return comparer enum1 Current  enum2 Current             It s untested  but it should do the job nonetheless  Note that what s particularly useful about this method is that it s full generic  i e  it can take two sequences of arbitrary  and different  types and return objects of any type   This solution of course assumes that you want to compare the nth item of seq1 with the nth item in seq2  If you want to do match the elements in the two sequences based on a particular property comparison  then you ll want to perform some sort of join operation  as suggested by danbruc using Enumerable Join  Do let me know if it neither of these approaches is quite what I m after and maybe I can suggest something else   Edit  Here s an example of how you might use the CompareSequences method with the comparer function you originally posted      Prints out to the console all the results returned by the comparer function  CompareTwoClass ReturnDifferences in this case   var results   CompareSequences list1  list2  CompareTwoClass ReturnDifferences   int index       foreach var element in results        Console WriteLine   0  000   1    index    element ToString

User · Answer

but how do we find the equivalent class in the second List to pass to the method   below    This is your actual problem  you must have at least one immutable property  a id or something like that  to identify corresponding objects in both lists  If you do not have such a property you  cannot solve the problem without errors  You can just try to guess corresponding objects by searching for minimal or logical changes   If you have such an property  the solution becomes really simple   Enumerable Join     listA  listB     a   gt  a Id  b   gt  b Id      a  b    gt  CompareTwoClass ReturnDifferences a  b          thanks to you both danbruc and Noldorin for your feedback  both Lists will be the same   length and in the same order  so the method above is close  but can you modify this   method to pass the enum Current to the method i posted above    Now I am confused     what is the problem with that  Why not just the following   for  Int32 i   0  i  lt  Math Min listA Count  listB Count   i          yield return CompareTwoClass ReturnDifferences listA i   listB i        The Math Min   call may even be left out if equal length is guaranted     Noldorin s implementation is of course smarter because of the delegate and the use of enumerators instead of using ICollection

User · Answer

This solution produces a result list  that contains all differences from both input lists   You can compare your objects by any property  in my example it is ID  The only restriction is that the lists should be of the same type   var DifferencesList   ListA Where x   gt   ListB Any x1   gt  x1 id    x id                Union ListB Where x   gt   ListA Any x1   gt  x1 id    x id

User · Answer

I hope that I am understing your question correctly  but you can do this very quickly with Linq   I m assuming that universally you will always have an Id property   Just create an interface to ensure this   If how you identify an object to be the same changes from class to class  I would recommend passing in a delegate that returns true if the two objects have the same persistent id   Here is how to do it in Linq   List lt Employee gt  listA   new List lt Employee gt             List lt Employee gt  listB   new List lt Employee gt              listA Add new Employee     Id   1  Name    Bill              listA Add new Employee     Id   2  Name    Ted               listB Add new Employee     Id   1  Name    Bill Sr               listB Add new Employee     Id   3  Name    Jim               var identicalQuery   from employeeA in listA                              join employeeB in listB on employeeA Id equals employeeB Id                              select new   EmployeeA   employeeA  EmployeeB   employeeB             foreach  var queryResult in identicalQuery                        Console WriteLine queryResult EmployeeA Name               Console WriteLine queryResult EmployeeB Name

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