c# - Compare nullable types in Linq to Sql

Question

I have a Category entity which has a Nullable ParentId field. When the method below is executing and the categoryId is null, the result seems null however there are categories which has null ParentId value.

What is the problem in here, what am I missing?

public IEnumerable<ICategory> GetSubCategories(long? categoryId)
{
    var subCategories = this.Repository.Categories.Where(c => c.ParentId == categoryId)
        .ToList().Cast<ICategory>();

    return subCategories;
}

By the way, when I change the condition to (c.ParentId == null), result seems normal.

question from:https://stackoverflow.com/questions/586097/compare-nullable-types-in-linq-to-sql

Answer 1

Other way:

Where object.Equals(c.ParentId, categoryId)

or

Where (categoryId == null ? c.ParentId == null : c.ParentId == categoryId)