open C# source file with gedit, syntax highlighting does not work

OS Version: Ubuntu 20.04
Locale：zh_cn
Problem:
When opening a C# source code file in gedit text editor, syntax highlighting does not work, but when opening a .py file, syntax highlighting works normally. The C# syntax highlighting plugin has been installed.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gedit 3.36.2-0ubuntu1
ProcVersionSignature: Ubuntu 5.15.0-73.80~20.04.1-generic 5.15.98
Uname: Linux 5.15.0-73-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Jun 6 19:40:33 2023
InstallationDate: Installed on 2023-04-25 (41 days ago)
InstallationMedia: Ubuntu 20.04.5 LTS "Focal Fossa" - Release amd64 (20220831)
SourcePackage: gedit
UpgradeStatus: No upgrade log present (probably fresh install)