How to format \contentslabel in titletoc?

Question

so in making ToC, \contentslabel gives you the section number, and using \thecontentslabel give you the default format. The problem is that this number is in arabic, while I want to use roman. Titletoc gave \contentslabel[<format>] to reformat this, but it doesn't tell me the wanted format. Any way to reformat it to roman numbering ?

My code is :

\documentclass[oneside,12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{titlesec}
\usepackage{titletoc}
\titleformat{\chapter}[display]{\bfseries\upshape\filcenter}{CHAPTER \ \Roman{chapter}}{1ex}{}
\titlespacing*{\chapter} {0pt}{-10pt}{40pt}
\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername} \thecontentslabel \ }%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots
\chapter{lorem}
\chapter{ipsum}

As you can see I format my chapter with roman, but \thecontentslabel still gives arabic. Format I want :

CHAPTER I LOREM
CHAPTER II IPSUM

the default output:

CHAPTER 1 LOREM
CHAPTER 2 IPSUM

some of my tried format(all fail):

\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername} \contentslabel[\Roman{contentslabel}] \ }%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots
\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername} \contentslabel[\Roman{\contentslabel}] \ }%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots
\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername} \Roman{\thecontentslabel} \ }%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots
\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername} \Roman{thecontentslabel} \ }%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots

Answer 1

Here is a solution based on the TeX primitive \romannumeral:

\documentclass[oneside,12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{titlesec}
\usepackage{titletoc}
\titleformat{\chapter}[display]{\bfseries\upshape\filcenter}{CHAPTER \ \Roman{chapter}}{1ex}{}
\titlespacing*{\chapter} {0pt}{-10pt}{40pt}
\titlecontents{chapter}[0pt]{\bfseries}%above code
{\MakeUppercase{\chaptername\ \romannumeral \thecontentslabel}\enspace}%numbered chapter%
{}%numberless chapter%
{\mdseries\titlerule*[0.75em]{.}\bfseries\thecontentspage}%for dots
\begin{document}
\tableofcontents
\chapter{lorem}
\chapter{ipsum}%
\end{document}