Finding the duplicates in a big collection

Hello all,

I have a little problem.
I must find duplicates i a CollectionBase object.
Actually there are 3 properties that give the uniqueness of the records.

I am reading am XML file that gives me the collection of objects in the CollectionBase object.
Then i must "say/display" witch records are duplicated according to some TAG node values.

 "public class XmlEmployesCollection : CollectionBase"

The problem is that sometimes there are more than 15 000 objects in the XmlEmployesCollection.
What i need is some guidelines for completing this task; "Finding the duplicates in a big collection."

I am using .NET Framework v2.0 with C#.

Thanks in advance,
So.



barbuleaAsked:
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thuannguyConnect With a Mentor Commented:
You can use three Dictionary<> to store the objects. Let's consider a concrete example in which the three "KEYS" are Age, Salary and Name
      Dictionary<string, Employee> nameDict = new Dictionary<string, Employee>();
      Dictionary<int, Employee> ageDict = new Dictionary<int, Employee>();
      Dictionary<double, Employee> salaryDict = new Dictionary<double, Employee>();
      List<Employee> duplicateList = new List<Employee>();

      public void Add(Employee employee)
      {
         bool isDuplicate = true;
         if (!nameDict.ContainsKey(employee.Name))
         {
            isDuplicate = false;
            nameDict.Add(employee.Name, employee);
         }
         
         if (!ageDict.ContainsKey(employee.Age))
         {
            isDuplicate = false;
            ageDict.Add(employee.Age, employee);
         }
         
         if (!salaryDict.ContainsKey(employee.Salary))
         {
            isDuplicate = false;
            salaryDict.Add(employee.Salary, employee);
         }
         if (isDuplicate)
              duplicateList.Add(employee);//this object is duplicate, add it to the duplicate list
}

When you read an object from the Xml file, just use the Add method to add it to the container. In the Add method, we check if the three "KEYS" already exist. Since we only store the references to the objects in the three dictionary, the memory cost is not so much.
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raterusCommented:
Whenever I need to find duplicates, I pull out the trusty HashTable object and start adding values to it based on a "should be unique" key.  Before you add the value to the hashtable, make use of the ContainsKey method to see if you've already put it there.  If you have, you know you have a duplicate.
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AlexNekCommented:
For key of the 3 properties it is not so easy but you need only additional steps.
For one Key/it can be complex key too/ you have at least 2 methods
- Sort the collection by key and remove one of the same neighbour item
- When you build a collection make an additional map by key and don't add the items which are already in map
It can be binary sort with preventing item duplication too.
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