System.Xml.Serialization.XmlSerializationWriter.ListUsedPrefixes C# (CSharp) Method

        private HashSet<int> ListUsedPrefixes(Dictionary<string, string> nsList, string prefix)
        {
            var qnIndexes = new HashSet<int>();
            int prefixLength = prefix.Length;
            const string MaxInt32 = "2147483647";
            foreach (string alias in _namespaces.Namespaces.Keys)
            {
                string name;
                if (alias.Length > prefixLength)
                {
                    name = alias;
                    if (name.Length > prefixLength && name.Length <= prefixLength + MaxInt32.Length && name.StartsWith(prefix, StringComparison.Ordinal))
                    {
                        bool numeric = true;
                        for (int j = prefixLength; j < name.Length; j++)
                        {
                            if (!Char.IsDigit(name, j))
                            {
                                numeric = false;
                                break;
                            }
                        }
                        if (numeric)
                        {
                            Int64 index = Int64.Parse(name.Substring(prefixLength), NumberStyles.Integer, CultureInfo.InvariantCulture);
                            if (index <= Int32.MaxValue)
                            {
                                Int32 newIndex = (Int32)index;
                                qnIndexes.Add(newIndex);
                            }
                        }
                    }
                }
            }
            if (qnIndexes.Count > 0)
            {
                return qnIndexes;
            }
            return null;
        }