Question

I don’t know if I’m missing something here but isn’t it strange that my code below always raises an exception on List.Contains part although I know for sure that list contain that element:

using System;
using System.Linq;
using System.Collections.Generic;

class SomeClass
{
  public string param1 {get; private set;}
  public string param2 {get; private set;}

  private SomeClass(){}
  public SomeClass(string param1, string param2)
  {
    this.param1 = param1;
    this.param2 = param2;
  }
}

class SomeClass2
{
  private List<SomeClass> myList = new List<SomeClass>();

  public void Add(SomeClass someclass)
  {
    myList.Add(someclass);
  }

  public void Remove(SomeClass someClass)
  {
    // this part always rises an exception
    if(!myList.Contains(someClass))
      throw new System.ArgumentException("some error");
    else myList.Remove(someClass);
  }
}

class MainClass
{
  public static void Main (string[] args)
  {
    var _someClass = new SomeClass2();          
    _someClass.Add(new SomeClass("aaa", "bbb"));

    try
    {
      _someClass.Remove(new SomeClass("aaa", "bbb"));   
    }
    catch(Exception e)
    {
      Console.WriteLine(e.Message);
    }
  }
}

Answer 1

Quote from the documentation of the Contains method:

This method determines equality by using the default equality
comparer, as defined by the object’s implementation of the
IEquatable(Of T).Equals method for T (the type of values in the list).

So you could implement IEquatable<T> on your objects if you want the Contains method to determine if 2 instances of SomeClass are equal:

class SomeClass: IEquatable<SomeClass>
{
    public string param1 { get; private set; }
    public string param2 { get; private set; }

    private SomeClass() { }
    public SomeClass(string param1, string param2)
    {
        this.param1 = param1;
        this.param2 = param2;
    }

    public bool Equals(SomeClass other)
    {
        return param1 == other.param1 && param2 == other.param2;
    }
}

Another possibility is to implement a custom EqualityComparer<T>:

class SomeClassEqualityComparer : IEqualityComparer<SomeClass>
{
    private static readonly SomeClassEqualityComparer _instance = new SomeClassEqualityComparer();

    public bool Equals(SomeClass x, SomeClass y)
    {
        return x.param1 == y.param1 && x.param2 == y.param2;
    }

    public int GetHashCode(SomeClass obj)
    {
        unchecked
        {
            int hash = 17;
            hash = hash * 23 + obj.param1.GetHashCode();
            hash = hash * 23 + obj.param2.GetHashCode();
            return hash;
        }
    }

    public static IEqualityComparer<SomeClass> Instance
    {
        get { return _instance; }
    }
}

and then use the following overload of the Contains method:

if (!myList.Contains(someClass, SomeClassEqualityComparer.Instance))
    throw new System.ArgumentException("some error");