C# recursion with a collection output

Question

I am trying to use recursive function and output a list of the resulting values. this is the code I came up with but it gives me

Error (CS0161): 'Script_Instance.wrapper(int, int, Grasshopper.DataTree, System.Collections.Generic.List)': not all code paths return a value (line 87)

public List<int> wrapper(int br, int depth, DataTree<int> topo, List<double> vals)
{
    List<int> collection = new List<int>();
    collection.Add(br);
    if (depth > 0)
    {
        double[] area = new double[vals.Count - 1];
        for (int i = 0;i < topo.Branches[br].Count;i++) 
        {
            area[i] = vals[topo.Branch(br)[i]];
        }
        double Im = area.Max();

        int IofMaxArea = area.ToList().IndexOf(Im);
        collection.Add(IofMaxArea);
        //  wrapper(topo.Branch(br)[IofMaxArea], de, topo, vals);
        wrapper(topo.Branch(br)[IofMaxArea], depth-1, topo, vals);
    }
    if (depth == 0)
    {
        return collection;
    }
}

I am trying to improve a python script that works fine but too slow for my needs. This is the python code.

import Grasshopper.DataTree as ghdt
import ghpythonlib.components as ghcomp
import rhinoscriptsyntax as rs

newTree = ghdt[object]()

def wrapper(br,depth):
    if depth>0:
        area = []
        for k in range(topo.Branch(br).Count):
            ar = vals[topo.Branch(br)[k]]

            #polygonC = rs.CloseCurve(polygon)
            area.append(ar)

        IofMaxArea = area.index(max(area))
        collection.append([pts[topo.Branch(br)[IofMaxArea]],topo.Branch(br)[IofMaxArea]])
        #[topo.Branch(br)[IofMaxArea]
        wrapper(topo.Branch(br)[IofMaxArea],depth-1)
    else: return None

def isovister(b,d):

    global collection
    collection = []

    collection.append([pts[b],b])
    wrapper(b,d)
    return collection
if topo.Branch(item).Count !=0:
    results = isovister(item,de)
    a = tuple(x[0] for x in results)
    b = tuple(x[1] for x in results)
else:
    a = None
    b = None

This is within Rhino3d+Grasshopper.

Answer 1

This code here makes sure that any positive value of depth results in the method being called again:

if (depth > 0)
{
    ...
    ...
    wrapper(topo.Branch(br)[IofMaxArea], depth-1, topo, vals);
}

The only way you'll ever get beyond that if block is if depth is less than or equal to 0. There's no need for a separate validation of the value of depth after that.

So change this:

if (depth == 0)
{
    return collection;
}

To this:

return collection;

The compiler is complaining because you haven't specified what value to return if depth doesn't equal 0. Even though logically you know your method will eventually return a value, the compiler doesn't.