c＃LastIndexOf几句话

Question

我正在尝试在文本字符串中获取“hello world”的最后索引。

            string sourceText = "hello world. no yes what where hello world but not right in left hello world end of text";
            string searchIndex = "hello world";
            int index = sourceText.LastIndexOf(searchIndex);
            if (index != -1)
            {
                Console.Write(index);
            }

但它总是返回-1。