using System;

​

public abstract class AbstractBaseClass

{

    public abstract string  SomeUniqueProperty { get; }

    protected bool Equals(AbstractBaseClass other)

    {

        return Equals(this.SomeUniqueProperty, other.SomeUniqueProperty);

    }

​

    public override bool Equals(object obj)

    {

        if (ReferenceEquals(null, obj))

        {

            return false;

        }

​

        if (ReferenceEquals(this, obj))

        {

            return true;

        }

​

        return obj.GetType() == this.GetType() && this.Equals((AbstractBaseClass)obj);

    }

​

    public override int GetHashCode()

    {

        return (this.SomeUniqueProperty != null ? this.SomeUniqueProperty.GetHashCode() : 0);

    }

​

    public static bool operator ==(AbstractBaseClass a, AbstractBaseClass b)

    {

        var isANull = ReferenceEquals(null, a);

        var isBNull = ReferenceEquals(null, b);

        return (isANull && isBNull) || a?.Equals(b) ?? false;

    }

​

    public static bool operator !=(AbstractBaseClass a, AbstractBaseClass b)

    {

        return !(a == b);

    }

​

}

​

public class ChildClass1 : AbstractBaseClass

{

    public override string SomeUniqueProperty

    {

        get

        {

            return "Child1";

        }

    }

}

​

public class ChildClass2 : AbstractBaseClass

{

    public override string SomeUniqueProperty

    {

        get

        {

            return "Child2";

        }

    }

}

public class Program

{

    public static void Main()

    {

        AbstractBaseClass baseClass = null;

        if (baseClass == null)

        {

            Console.WriteLine("Null check succeeded...");

        }

        else

        {

            Console.WriteLine("Null check failed...");

        }

    }

}