当我定义IEqualityComparer时为什么不起作用<T>

Question

If I try to do this - my call to oldList.Except( newList ) seems to return all items - no comparison is working.

   List<ControlAndTopLevelControlPair> oldOnly = oldList.Except( newList ).ToList();

   public class ControlAndTopLevelControlPair : IEqualityComparer<ControlAndTopLevelControlPair>
   {
      public int CONTROLOI { get; set; }
      public int VIEWCONTROL_OI { get; set; }
      public bool Equals( ControlAndTopLevelControlPair x, ControlAndTopLevelControlPair y )
      {
         return x.CONTROLOI.Equals( y.CONTROLOI ) && x.VIEWCONTROL_OI.Equals( y.VIEWCONTROL_OI );
      }

      public int GetHashCode( ControlAndTopLevelControlPair obj )
      {
         return obj.CONTROLOI.GetHashCode( ) ^ obj.VIEWCONTROL_OI.GetHashCode( );
      }

But if I define a custom comparer then the following works:

List<ControlAndTopLevelControlPair> oldOnly = oldList.Except( newList, new ControlAndTopLevelControlPairComparer() ).ToList( );

   public class ControlAndTopLevelControlPairComparer : IEqualityComparer<ControlAndTopLevelControlPair>
   {
      public bool Equals( ControlAndTopLevelControlPair x, ControlAndTopLevelControlPair y )
      {
         return x.CONTROLOI.Equals( y.CONTROLOI ) && x.VIEWCONTROL_OI.Equals( y.VIEWCONTROL_OI );
      }

      public int GetHashCode( ControlAndTopLevelControlPair obj )
      {
         return obj.CONTROLOI.GetHashCode() ^ obj.VIEWCONTROL_OI.GetHashCode();
      }
   }

Answer 1

When you don't specify a Comparer, it will default to calls to GetHashCode and Equals(object) , which you don't implement.

If you put a breakpoint in your methods, you'll see they're never called. If you do the same with your implementations of GetHashCode() and Equals(object) , you'll see those are called.

public class ControlAndTopLevelControlPair 
{
  public int CONTROLOI { get; set; }
  public int VIEWCONTROL_OI { get; set; }

  public override int GetHashCode() {  return CONTROLOI ^ VIEWCONTROL_OI; }
  public override bool Equals (object other) 
  {
     // your implementation
  }
}

Answer 2

The IEqualityComparer is the interface in which you define an object who's sole job is to compare some other objects .

You use the IEquatable object to define how an object compares itself for equality with another object. If you do that, then you don't need to create a new comparer object and pass it into Except .

Ensure that whenever you implement IEquatble that you override the object's GetHashCode method as well.

public class ControlAndTopLevelControlPair : IEquatable<ControlAndTopLevelControlPair>
{
    public int CONTROLOI { get; set; }
    public int VIEWCONTROL_OI { get; set; }
    public bool Equals(ControlAndTopLevelControlPair other)
    {
        if (other == null) return false;
        return CONTROLOI.Equals(other.CONTROLOI)
            && VIEWCONTROL_OI.Equals(other.VIEWCONTROL_OI);
    }
    public override bool Equals(object obj)
    {
        return Equals(obj as ControlAndTopLevelControlPair);
    }
    public override int GetHashCode()
    {
        return CONTROLOI.GetHashCode() ^ VIEWCONTROL_OI.GetHashCode();
    }
}

If you're going to override GetHashCode you should also override the object's Equals method so that the two definitions of equality stay in sync.

Answer 3

You need to override the Equals and GetHashCode in the first case.

public class ControlAndTopLevelControlPair {
  public int CONTROLOI { get; set; }
  public int VIEWCONTROL_OI { get; set; }
  public override bool Equals(object x)
  {
     ControlAndTopLevelControlPair c = x as ControlAndTopLevelControlPair;
     if(c == null) return false;
     return c.CONTROLOI.Equals(CONTROLOI) && c.VIEWCONTROL_OI.Equals(VIEWCONTROL_OI);
  }

  public override int GetHashCode()
  {
     return CONTROLOI.GetHashCode( ) ^ VIEWCONTROL_OI.GetHashCode( );
  }
}

Note the override keywords, the class doesn't need to implement IEqualityComparer