Can someone tell me why this simple bit of c# code won't work, involves recursively calling method from within itself to get the root category ID

Question

All root categories in table have a PCATID of null. The method actually does the job, sets cat to the very root category, but then amazingly fails to return it once it hits return cat; does some weird jumping in and out of the "if" statement, then again to GetRootCat(cat); and returns not the root category, but the first parent category of cat.

Call:

listModel.PCATID = GetRootCat(cat).CategoryID;

Method:

    private Category GetRootCat(Category cat)
    {

        if (cat.PCATID != null)
        {
            cat = repository.Categories.FirstOrDefault(x => x.CategoryID == cat.PCATID);
            GetRootCat(cat);

        }

            return cat;
    }

Answer 1

Replace

GetRootCat(cat); 

with

return GetRootCat(cat);

Answer 2

It doesn't "fail to return", it returns to itself because it's a recursive call. But when it calls itself, it throws away the return value, and falls through to the return cat , which gives you the parent of the category you called it with.

This would be more sane:

private Category GetRootCat(Category cat) {
    if (cat.PCATID == null) { /* This is the root */
      return cat;
    } else {
        Category parent = repository.Categories.FirstOrDefault(x => x.CategoryID == cat.PCATID);
        return GetRootCat(parent);
    }
}