Question

I’m trying to perform a good comparison when I use List.Contains(T item).

The problem is that I’m using BaseItem as a list item. And I need to verify if one object inside of the list has the same properties values that the object which I plan to add.

For example:

public abstract class BaseItem
{
    // some properties

    public override bool Equals(object obj)
    {
        return obj != null && this.GetType() == obj.GetType();
    }
}

public class ItemA : BaseItem
{
    public int PropertyA { get; set; }

    public override bool Equals(object obj)
    {
        if (base.Equals(obj) == false)
            return false;

        return (this.PropertyA == (obj as ItemA).PropertyA;
    }
}

public class ItemB : BaseItem
{
    public int PropertyB { get; set; }

    public override bool Equals(object obj)
    {
        if (base.Equals(obj) == false)
            return false;

        return this.PropertyB == (obj as ItemB).PropertyB;
    }
}

public class Program
{
    static void Main(string[] args)
    {
        List<BaseItem> items = new List<BaseItem>()
        {
            new ItemB() { PropertyB = 3 },
            new ItemA() { PropertyA = 2 },
            new ItemB() { PropertyB = 2 }
        };

        BaseItem newItem = new ItemA() { PropertyA = 2 };
        items.Contains(newItem);  // should return 'True', because the first element is equals than 'newItem'
    }
}

I’m not sure if it’s correct to override Equals method or should I have to implement IEquality interface.

Answer 1

The standard way to test for collection membership (list membership, hashset membership, …) is to use the .Contains method, which in turn uses IEquatable<T>.Equals. The default implementation of IEquatable<T>.Equals (if you do not provide your own implementation) uses Object.Equals(Object)).

You can certainly implement IEquatable<T>. This will improve performance of the Contains check slightly. See msdn.microsoft.com/en-us/library/ms131190.aspx.

If you implement IEquatable<T> you should still override Object.Equals and Object.GetHashCode to provide consistent behavior

If you implement Equals, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behavior is consistent with that of the IEquatable.Equals method.

http://msdn.microsoft.com/en-us/library/ms131190.aspx

See this question for a related discussion

Simplify Overriding Equals(), GetHashCode() in C# for Better Maintainability