Comparing to Object in C#

Question

Hello I am new in C# and i have a little question : when I call the method list1.contains(product) is it like java we need to override the Equals method ?

if yes ? is this the way to do it ?

public partial class product 
{
    public product()
    {
    }

    public int idProduct { get; set; }

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

        product other = (product)obj;
        if (idProduct != other.idProduct)
        {
            return false;
        }
        return true;
    }
}

Answer 1

In addition to @driis answer, you can also create a new IEqualityComparer<Product> like:

public sealed class ProductEqualityComparer : IEqualityComparer<Product>
{
    public bool Equals(Product x, Product y)
    {
        if (ReferenceEquals(x, y)) return true;
        if (ReferenceEquals(x, null)) return false;
        if (ReferenceEquals(y, null)) return false;
        if (x.GetType() != y.GetType()) return false;
        return x.idProduct == y.idProduct;
    }

    public int GetHashCode(Product obj)
    {
        return obj.idProduct;
    }
}

And then you can pass that Comparer to Contains like:

if (list1.Contains(product, new ProductEqualityComparer()))
{

}

Where list1 is List<Product> defined as:

Product product = new Product { idProduct = 1 };
List<Product> list1 = new List<Product>
{
    new Product{idProduct =  1},
    new Product{idProduct = 2}
};

Or you can inherit from EqualityComparer<T> and then override Equals and GetHashCode, because MSDN states

We recommend that you derive from the EqualityComparer<T> class instead of implementing the IEqualityComparer<T> interface, because the EqualityComparer<T> class tests for equality using the IEquatable<T>.Equals method instead of the Object.Equals method. This is consistent with the Contains, IndexOf, LastIndexOf, and Remove methods of the Dictionary<TKey, TValue> class and other generic collections.

public sealed class ProductEqualityComparer : EqualityComparer<Product>
{
    public override bool Equals(Product x, Product y)
    {
        if (ReferenceEquals(x, y)) return true;
        if (ReferenceEquals(x, null)) return false;
        if (ReferenceEquals(y, null)) return false;
        if (x.GetType() != y.GetType()) return false;
        return x.idProduct == y.idProduct;
    }

    public override int GetHashCode(Product obj)
    {
        return obj.idProduct;
    }
}

Answer 2

It's probably worth mentioning that you can shorten your code a bit by implementing IEquatable<T>.

List<T>.Contains will call IEquatable<T>.Equals if the type in your generic list implements IEquatable, avoiding the cast:

public class product : IEquatable<product>
{
    public int idProduct { get; set; }

    // List<product>.Contains will call IEquatable<product>.Equals if available.
    public bool Equals(product other)
    {
        return ReferenceEquals(other, this) || other.idProduct == this.idProduct;
    }

    // Note: you should still override object.Equals.
    public override bool Equals(object other)
    {
        return this.Equals(other as product);
    }
}

Also in C# classes are typically PascalCased and not camelCased.

Answer 3

Yes, you should override Equals to provide equality semantics for your class. By default, reference equality will be in play.

Your Equals method will work, but in C# we can also override the == operator. Therefore, it might not be safe to use in an Equals method. It would be better to use ReferenceEquals for the reference comparisons:

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(obj,null))
        {
            return false;
        }
        if (ReferenceEquals(this,obj))
        {
            return true;
        }
        if (this.GetType() != obj.GetType())
        {
            return false;
        }
        product other = (product)obj;
        if (idProduct != other.idProduct)
        {
            return false;
        }
        return true;
    }