Simple Question IEqualityComparer <T>

Question

I am trying to remove duplicate entries from a list that contains a shared object.

public class MessageInfo { public DateTime Date { get; set; } public string To { get; set; } public string Message { get; set; } } public class SMSDupeRemover : IEqualityComparer<MessageInfo> { public bool Equals(MessageInfo x, MessageInfo y) { throw new NotImplementedException(); } public int GetHashCode(MessageInfo obj) { throw new NotImplementedException(); } } 

And the code to remove duplicates:

 IEnumerable<MessageInfo> new_texts = text_messages.Distinct(new SMSDupeRemover()); 

Problem: Equals and GetHashCode are never called. Does anyone know why?

Answer 1

Because Distinct lazy. Try adding ToList() to the end.

Longer answer. Linq's operations are actually declarative. They define the request, but they do not say to fulfill it. IEnumerable<T> contains no data, just a request definition. You created a query, ok, how to get the data?

• foreach IEnumerable . Since foreach is required, all data must be restored (request completed).
• Call ToList / ToDictionary . These collections store real data, so the system must complete the request to fill them.

Answer 2

LINQ is lazily evaluated, so it won’t run until you name GetEnumerator and possibly even MoveNext .

Try adding .ToList() to the end of this query, and it should execute right away.

Answer 3

IEqualityComparer is not called until IEnumerable is listed.

Try

 var new_texts = text_messages.Distinct(new SMSDupeRemover()).ToList(); 

Answer 4

I would do

 List<MessageInfo> unique_msgs = new List(text_messages.Distinct(new SMSDupeRemover())); 

to immediately display the new list. Unless you really need an enumerator to save memory.