C# dictionary lookup using derived class

Question

I think this could be a common question but I failed to find any post about it.

I'm writing some pseudo code below. If you want to compile the code, please just ignore this post.

So say two classes: one base class and one child class. NOTE: both classes have override Equals() and GetHashCode() function to ensure equality with same property.

public class A // A has a string property of name
public class B:A // B has a string property of title

var a = new A{name = "bob"};
var b = new B{name = "bob", title = "em"};

Some code have a dictionary based on A

var dict = new Dictionary<A>();

Doing some adding stuff, for instance,

dict.Add(a);

However, the lookup function will raise KeyNotFoundException if i use a derived class searching with/o type cast

dict[b];

Dictionary will calculate the hashcode of B instead of A and raised the exception according to that.

A simple and awkward solution is to create a new instance of A based on B's property.

dict[new A{name = b.name}];

I wonder if there is any better solution?

Answer 1

Try creating an EqualityComparer, and pass an instance of it in the constructor of the dictionary.

class AEqualityComparer : IEqualityComparer<A>
{
    public bool Equals(A x, A y)
    {
        return x.Equals(y);
    }

    public int GetHashCode(A obj)
    {
        return obj.GetHashCode();
    }
}

var dict = new Dictionary<A, object>(new AEqualityComparer());