VS2017 implicitly typed local variables are not available in C-sharp2?

VS2017 Enterprise Edition, var do not recognize? How to upgrade C-sharp?
No problem generating DLL.

Vs2017 c-sharp
Mar.22,2021

(1) right-click the current project, click "Properties]
(2) Click" generate "in the left Tab, and click" Advanced "in the lower right corner of the interface.
(3) modify the language version

requires C-sharp3 or higher version

right-click the project, and then select" Properties ". In the pop-up Properties dialog box, In the 'applications' tab, change the. NET Framework version of the current project to version 3.0 or above.

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-42b80b5-688e.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-42b80b5-688e.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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