Question

I have a class:

public abstract class AbstractDictionaryObject
    {
        public virtual int LangId { get; set; }

        public override bool Equals(object obj)
        {
            if (obj == null || obj.GetType() != GetType())
            {
                return false;
            }

            AbstractDictionaryObject other = (AbstractDictionaryObject)obj;
            if (other.LangId != LangId)
            {
                return false;
            }

            return true;
        }

        public override int GetHashCode()
        {
            int hashCode = 0;               
            hashCode = 19 * hashCode + LangId.GetHashCode();
            return hashCode;
        }

And I have derived classes:

public class Derived1:AbstractDictionaryObject
{...}

public class Derived2:AbstractDictionaryObject
{...}

In the AbstractDictionaryObject is only one common field: LangId.
I think this is not enough to overload methods (properly).
How can I identify objects?

Answer 1

For one thing you can simplify both your methods:

 public override bool Equals(object obj)
 {
     if (obj == null || obj.GetType() != GetType())
     {
         return false;
     }

     AbstractDictionaryObject other = (AbstractDictionaryObject)obj;
     return other.LangId == LangId;
 }

 public override int GetHashCode()
 {
     return LangId;
 }

But at that point it should be fine. If the two derived classes have other fields, they should override GetHashCode and Equals themselves, first calling base.Equals or base.GetHashCode and then applying their own logic.

Two instances of Derived1 with the same LangId will be equivalent as far as AbstractDictionaryObject is concerned, and so will two instances of Derived2 – but they will be different from each other as they have different types.

If you wanted to give them different hash codes you could change GetHashCode() to:

 public override int GetHashCode()
 {
     int hash = 17;
     hash = hash * 31 + GetType().GetHashCode();
     hash = hash * 31 + LangId;
     return hash;
 }

However, hash codes for different objects don’t have to be different… it just helps in performance. You may want to do this if you know you will have instances of different types with the same LangId, but otherwise I wouldn’t bother.