public class CsnuService : ICsnuService
{
    private readonly HttpClient _httpClient;

    public CsnuService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<IEnumerable<XElement>> GetIndividualElementsAsync(string url)
    {
        if (string.IsNullOrEmpty(url))
        {
            throw new ArgumentException("URL cannot be empty.", nameof(url));
        }

        using var response = await _httpClient.GetStreamAsync(url);
        using var reader = XmlReader.Create(response);
        return GetIndividualElements(reader);
    }

    public IEnumerable<XElement> GetIndividualElements(XmlReader reader)
    {
        while (reader.Read())
        {
            if (reader.NodeType == XmlNodeType.Element && reader.Name == "INDIVIDUALS")
            {
                while (reader.ReadToFollowing("INDIVIDUAL"))
                {
                    yield return (XElement)XElement.ReadFrom(reader);
                }
                
            }
        }
    }
}

public class CsnuService : ICsnuService
{
    private readonly HttpClient _httpClient;

    public CsnuService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<IEnumerable<XElement>> GetIndividualElementsAsync(string url)
    {
        if (string.IsNullOrEmpty(url))
        {
            throw new ArgumentException("URL cannot be empty.", nameof(url));
        }

        using var response = await _httpClient.GetStreamAsync(url);
        using var reader = XmlReader.Create(response);
        return GetIndividualElements(reader);
    }

    public IEnumerable<XElement> GetIndividualElements(XmlReader reader)
    {
        while (reader.Read())
        {
            if (reader.NodeType == XmlNodeType.Element && reader.Name == "INDIVIDUALS")
            {
                while (reader.ReadToFollowing("INDIVIDUAL"))
                {
                    yield return (XElement)XElement.ReadFrom(reader);
                }
                
            }
        }
    }
}