/warn (C# Compiler Options)
The /warn option specifies the warning level for the compiler to display.
The warning level you want displayed for the compilation: Lower numbers show only high severity warnings; higher numbers show more warnings. Valid values are 0-4:
Turns off emission of all warning messages.
Displays severe warning messages.
Displays level 1 warnings plus certain, less-severe warnings, such as warnings about hiding class members.
Displays level 2 warnings plus certain, less-severe warnings, such as warnings about expressions that always evaluate to true or false.
4 (the default)
Displays all level 3 warnings plus informational warnings.
To get information about an error or warning, you can look up the error code in the Help Index. For other ways to get information about an error or warning, see C# Compiler Errors.
/w is the short form of /warn.
To set this compiler option in the Visual Studio development environment
Open the project's Properties page.
Click the Build property page.
Modify the Warning Level property.
For information on how to set this compiler option programmatically, see WarningLevel.
Compile in.cs and have the compiler only display level 1 warnings:
csc /warn:1 in.cs