Question

I keep getting a null reference when I try to create a collection and at the same time add a category to it. Did I forgot something? Or am I doing it all wrong?

private static void SamepleMethod(ICollectionRepository collectionRepo)
    {
        Collection collection = new Collection { CollectionName = "Collection" };

        Category category = new Category { Collection = collection, CategoryName = "Category" };

        category.SetCollection(collection);
        collection.AddCategory(category);

        collectionRepo.Save(collection);
    }

Collection:

public class Collection
{
    public virtual int? Id { get; set; }
    public virtual string CollectionName { get; set; }
    public virtual ICollection<Category> Categories { get; set; }

    public virtual void AddCategory(Category category)
    {
        Categories.Add(category);
        category.Collection = this;
    }
}

Category:

public class Category
{
    public virtual int? Id { get; set; }
    public virtual string CategoryName { get; set; }
    public virtual Collection Collection { get; set; }

    public virtual void SetCollection(Collection collection)
    {
        Collection = collection;
        collection.Categories.Add(this);
    }
}

Collection.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               namespace="ACME.Model"
               assembly="ACME.Model">

<class name="Collection" table="Collections" >

<id name="Id" column="CollectionId" type="int" >
  <generator class="identity" />
</id>

<set name="Categories" inverse="true" cascade="all">
  <key column="CollectionId" />
  <one-to-many class="Category"></one-to-many>
</set>

<property name="CollectionName">
  <column name="CollectionName" sql-type="nvarchar(50)" />
</property>

Category.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               namespace="ACME.Model"
               assembly="ACME.Model">

<class name="Category" table="Categories" >

<id name="Id" column="CategoryId" type="int" >
  <generator class="identity" />
</id>

<many-to-one name="Collection"
             class="Collection"
             column="CollectionId"
             not-null="true" />

<property name="CategoryName">
  <column name="CategoryName" sql-type="nvarchar(50)" />
</property>

Answer 1

you never initialized the Categories property in your Collection class, and therefore it remains at its default value of null…
You may want to initialize it to a new collection in the constructor.

Also, I’ve counted at least 3 different code lines where you associate a category with a collection.
I believe once is enough 🙂 (I like it best in the AddCategory method, or in the ctor of Category).

Another small thing- hbm mapping files are a thing of the past; nowadays it’s much more convenient (and less error-prone) to map in code.
You can do it either using the old extension fluent nHibernate, or the new official mapping-by-code, which is also very good, but alas- lacking in documentation.