關鍵字是保留字，在程式程式碼中不能將其用作變數名稱，常數名稱等。
在 C# 關鍵字不能用作識別符號。但是，如果要使用關鍵字作為識別符號，我們可以使用@字元加到關鍵字的前面，如：@class, @private等。
C# 程式設計語言中可用的保留關鍵字如下列表：
| abstract | base | as | bool | break | catch | case |
|---|---|---|---|---|---|---|
| byte | char | checked | class | const | continue | decimal |
| private | protected | public | return | readonly | ref | sbyte |
| explicit | extern | false | finally | fixed | float | for |
| foreach | goto | if | implicit | in | in (generic modifier) | int |
| ulong | ushort | unchecked | using | unsafe | virtual | void |
| null | object | operator | out | out (generic modifier) | override | params |
| default | delegate | do | double | else | enum | event |
| sealed | short | sizeof | stackalloc | static | string | struct |
| switch | this | throw | true | try | typeof | uint |
| abstract | base | as | bool | break | catch | case |
| volatile | while | - | - | - | - | - |