C#: Is pragma warning restore needed?

Question

From msdn I get this:

#pragma warning disable warning-list
#pragma warning restore warning-list

In the examples, both disable and restore are used. Is it necessary to restore if I want it disabled for a whole file?

Like, if I do not restore, how far does it carry? Are the warnings disabled for everything compiled after that? Or just for the rest of that file? Or is it ignored?

Answer 1

If you do not restore the disabling is active for the remainder of the file.

Interestingly this behaviour is not defined in the language specification. (see section 9.5.8) However the 9.5.1 section on Conditional compilation symbols does indicate this "until end of file behaviour"

The symbol remains defined until a #undef directive for that same symbol is processed, or until the end of the source file is reached.

Given the 'pre-processor' is actually part of the lexical analysis phase of compilation it is likely that this behaviour is an effective contract for Microsoft's and all other implementations for the foreseeable future (especially since the alternate would be hugely complex and non deterministic based on source file compilation order)

Answer 2

No, you'll find that the compiler will automatically restore any disabled warning once it's finished parsing a source file.

#pragma warning disable 649
struct MyInteropThing
{
    int a;
    int b;
}
#pragma warning restore 649

In the above example I've turned of warning CS00649 because I intend to use this struct in an unsafe manner. The compiler will not realize that I will be writing to memory that has this kind of layout so I'll want to ignore the warning:

Field 'field' is never assigned to, and will always have its default value 'value'

But I don't want the entire file to not be left unchecked.

Answer 3

Let's say I have a private field that is initialized using reflections, the compiler obviously can't find any code directly writing into this field so it will show a warning - that I don't want to show.

Let's also say I have another private field defined 3 lines below the first that I forgot to initialize, if I disable the warning for the entire file this will not trigger a warning.

So, the best usage for #pragma warning is to put a "warning disable" right before the line that causes the warning that I want to suppress and a "warning restore" right after the line so the same condition in a different location in the file will still trigger a warning.