The out parameter does not work in C#. Why?

Question

The out parameter does not work in C#. Why?

Here is my code:

using System;
using System.Collections.Generic;
                    
public class Program
{
    public static void Main()
    {
        var hashSet = new HashSet<int>(2);
        Console.WriteLine(hashSet.Contains(2));
    }
    
    public ListNode ReverseList(ListNode head) 
    {
        helper(head, out var newHead);
        return newHead;
    }
    
    private ListNode helper(ListNode current, out ListNode newHead) 
    {
        newHead = null;
        
        if (current == null) 
        {
            return null;
        }
        
        var next = helper(current.next, out newHead);

        if (next == null) 
        {
            newHead = current;
        } 
        else 
        {
            next.next = current;
        }
        
        return current;
    }
}

public class ListNode 
{
    public int val;
    public ListNode next;

    public ListNode(int val = 0, ListNode next = null) 
    {
        this.val = val;
        this.next = next;
    }
}

Here is the error I am getting:

Compilation error (line 13, col 30): ) expected
Compilation error (line 13, col 37): ; expected
Compilation error (line 13, col 37): Invalid expression term ')'

I have spent 2 hours straight trying to figure out what is wrong. Help me, please.

Here is my code online.

Answer 1

I tried your sample at dotnet fiddle and it seems like the compiler (version) matters. I got the same exception when running your code on the .NET 4.7.2 compiler, but when you change the compiler to Roslyn 3.8 or .NET 5 the sample works.