返回时，完全停止递归

Question

I do recursion to find a long value within a List with multiple children that also can have children.

following method:

public TaxonomyData getTaxonomyData(long taxId, List<TaxonomyData> TaxonomyTree, TaxonomyData output)
{
    //find taxid in the taxonomy tree list and return the taxonomydata

    foreach (TaxonomyData td in TaxonomyTree)
    {
        if (td.TaxonomyId == taxId)
        {
                output = td;
                //return td; => when doing a return here means I already found a match so it is not necessary to do all the recursion.
        }
        else if (td.Taxonomy.Length > 0)
        {
            getTaxonomyData(taxId, td.Taxonomy.ToList(), output);
        }
    }

    return output;
}

Is it possible when I do return td; (see commented row) that my whole recursion stops?

Thanks

Answer 1

I suspect you want something like:

public TaxonomyData GetTaxonomyData(long taxId, IEnumerable<TaxonomyData> tree)
{
    foreach (TaxonomyData td in tree)
    {
        if (td.TaxonomyId == taxId)
        {
            return td;
        }
        else
        {
            // See if it's in the subtree of td
            TaxonomyData data = GetTaxonomyData(taxId, td.Taxonomy);
            if (data != null)
            {
                return data;
            }
        }
    }
    // Haven't found it anywhere in this tree
    return null;
}

Each return only returns one level, but by checking the return value in the else clause, we can return all the way up the stack when we find the right value.

The final result returned to the caller will be a null reference if it hasn't been found.

Note that I've removed the "output" parameter, which wouldn't have been effective anyway as it wasn't a ref parameter, and isn't as clear as just using the return value.

Answer 2

A linq extension solution i came up with, probably slower but there you go..

public static class Ext
{
    public static T SingleOrDefault<T>(this IEnumerable<T> enumerable,Func<T,bool> predicate, Func<T,T> defaultSelector)
        where T : class
    {   
        return enumerable.SingleOrDefault(predicate) ?? enumerable.SkipWhile<T>(t=>defaultSelector(t) == null).Select(defaultSelector).SingleOrDefault();
    } 

    public static TaxonomyData Get(this IEnumerable<TaxonomyData> tree, int taxId)
    {
        return tree.SingleOrDefault(t=> t.TaxonomyId == taxId,t=>t.Taxonomy.Get(taxId));
    }
}