Question

It seems that a many to many relationship should always be bidirectional since each side is probably interested in accessing the other side’s collection.
Does anyone have a real world example where a many to many relationship made sense being unidirectional.

Answer 1

A simple example,

// a Product can belong to many ProductCollections
class Product
{
    // some properties
}

// a ProductCollection can contain many Products
class ProductCollection
{
    public IList<Product> Products { get; private set; }    
    // ...
    // other collection properties
}

You may want to group/organise products in generic collections and allow the end user to create new collections.

Some collections are used to display products on a website, others are used for administration (person A is responsible for… , person B for other products),
some products are keps in a ‘out of stock’ collection, others in a ‘need to be updated’ collection, etc…

In such a scenario you will most likely always want to retrieve a products using a specific collection.

And if you need all the collections a product belongs to, then you can always write a function for that.