while
because it's a C# keyword. The following table contains the C# keywords.
abstract | as | base | bool | break |
byte | case | catch | char | checked |
class | const | continue | decimal | default |
delegate | do | double | else | enum |
event | explicit | extern | false | finally |
fixed | float | for | foreach | goto |
if | implicit | in | int | interface |
internal | is | lock | long | namespace |
new | null | object | operator | out |
override | params | private | protected | public |
readonly | ref | return | sbyte | sealed |
short | sizeof | stackalloc | static | string |
struct | switch | this | throw | true |
try | typeof | uint | ulong | unchecked |
unsafe | ushort | using | using static | virtual |
void | volatile | while |