You have to use constants within a class.  So like this:

public const int MY_NUMBER = 9;

You’d need to put it in a class somewhere, and the usage would be ClassName.MY_NUMBER

Number Defines

public const int SOFTWARE_VERSION = 42;
const int SOFTWARE_VERSION = 42;

String Defines

public const string MY_THINGS_PATH = "c:\\windows\\";
const string MY_THINGS_PATH = "c:\\windows\\";
private const string MY_THINGS_PATH = "c:\\windows\\";