How to use the IEqualityComparer

Question

I have some bells in my database with the same number. I want to get all of them without duplication. I created a compare class to do this work, but the execution of the function causes a big delay from the function without distinct, from 0.6 sec to 3.2 sec!

Am I doing it right or do I have to use another method?

reg.AddRange(     (from a in this.dataContext.reglements      join b in this.dataContext.Clients on a.Id_client equals b.Id      where a.date_v <= datefin && a.date_v >= datedeb      where a.Id_client == b.Id      orderby a.date_v descending       select new Class_reglement      {          nom  = b.Nom,          code = b.code,          Numf = a.Numf,      })     .AsEnumerable()     .Distinct(new Compare())     .ToList());  class Compare : IEqualityComparer<Class_reglement> {     public bool Equals(Class_reglement x, Class_reglement y)     {         if (x.Numf == y.Numf)         {             return true;         }         else { return false; }     }     public int GetHashCode(Class_reglement codeh)     {         return 0;     } } 

Answer 1

Your GetHashCode implementation always returns the same value. Distinct relies on a good hash function to work efficiently because it internally builds a hash table.

When implementing interfaces of classes it is important to read the documentation, to know which contract you’re supposed to implement.¹

In your code, the solution is to forward GetHashCode to Class_reglement.Numf.GetHashCode and implement it appropriately there.

Apart from that, your Equals method is full of unnecessary code. It could be rewritten as follows (same semantics, ¼ of the code, more readable):

public bool Equals(Class_reglement x, Class_reglement y) {     return x.Numf == y.Numf; } 

Lastly, the ToList call is unnecessary and time-consuming: AddRange accepts any IEnumerable so conversion to a List isn’t required. AsEnumerable is also redundant here since processing the result in AddRange will cause this anyway.

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¹ Writing code without knowing what it actually does is called cargo cult programming. It’s a surprisingly widespread practice. It fundamentally doesn’t work.

Answer 2

Try This code:

public class GenericCompare<T> : IEqualityComparer<T> where T : class {     private Func<T, object> _expr { get; set; }     public GenericCompare(Func<T, object> expr)     {         this._expr = expr;     }     public bool Equals(T x, T y)     {         var first = _expr.Invoke(x);         var sec = _expr.Invoke(y);         if (first != null && first.Equals(sec))             return true;         else             return false;     }     public int GetHashCode(T obj)     {         return obj.GetHashCode();     } } 

Example of its use would be

collection = collection     .Except(ExistedDataEles, new GenericCompare<DataEle>(x=>x.Id))     .ToList();