private static void Foo( System.Object obj ) {
    if ( null == obj ) {
        throw new System.NullArgumentException( "obj" );
    }
    // do something with obj
}
private static void Bar( System.Object obj ) {
#if DEBUG
    if ( null == obj ) {
        throw new System.NullArgumentException( "obj" );
    }
#endif
    // do something with obj
}