How to mark a method will throw unconditionally?

Question

Is there a way to decorate a method that will do some logging, then throw an exception unconditionally, as such?

I have code like this:

void foo(out int x)
{
  if( condition() ) { x = bar(); return; }

  // notice that x is not yet set here, but compiler doesn't complain

  throw new Exception( "missed something." );
}

If I try writing it like this I get a problem:

void foo(out int x)
{
  if( condition() ) { x = bar(); return; }

  // compiler complains about x not being set yet

  MyMethodThatAlwaysThrowsAnException( "missed something." );
}

Any suggestions? Thanks.

"x has out attribute and hasn't been set yet at the end of method" — k0dek0mmand0, Oct 05 '09 at 14:52
I'm confused - how is it thrown unconditionally if it doesn't get thrown when x is set (and a return is done) — Matt, Oct 05 '09 at 14:53
I want to mark MyMethodThatAlwaysThrowsAnException as a method that always throws an exception, not the foo method. — k0dek0mmand0, Oct 05 '09 at 14:54
I wonder where that answer with the guard clause went. I thought it was a better answer to just reverse the logic. — Brandon, Oct 05 '09 at 14:55
he probably want's a common throw instead of duplicate code for an unset value. but that's my guess. (people need to remeber their way is not the only way) — Matthew Whited, Oct 05 '09 at 14:57
If I understand this correctly, you're trying to refactor out duplicate code because this is a pattern you're using in multiple places. Instead of `Log("Missed Something"); throw new Exception("Missed Something")` you want a single method call. — Greg, Oct 05 '09 at 15:00
@Greg: something like that. I also want to add a Debugger.Break statement in the MyThrow as I find it convenient. — k0dek0mmand0, Oct 05 '09 at 15:06
Does this answer your question? [declare a method always throws an exception?](https://stackoverflow.com/questions/3892938/declare-a-method-always-throws-an-exception) — MuiBienCarlota, Dec 04 '19 at 13:54

score 21 · Accepted Answer · answered Oct 05 '09 at 14:54

How about this?

bool condition() { return false; }
int bar() { return 999; }
void foo(out int x)
{
    if (condition()) { x = bar(); return; }
    // compiler complains about x not being set yet 
    throw MyMethodThatAlwaysThrowsAnException("missed something.");
}
Exception MyMethodThatAlwaysThrowsAnException(string message)
{
    //this could also be a throw if you really want 
    //   but if you throw here the stack trace will point here
    return new Exception(message);
}

score 4 · Answer 2 · answered Dec 18 '12 at 14:43

4

It's a very old thread but I just want to add you should write it different from the start:

void foo(out int x)
{
    if (!condition()) 
        MyMethodThatAlwaysThrowsAnException("missed something.");

    x = bar();
    // and so on...
}

That way compiler won't complain and your code is much more clear.

answered Dec 18 '12 at 14:43

Matthew Bednarski

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3

The actual code is probably more complex, so that your suggestion may not make sense. – avl_sweden Mar 01 '16 at 15:15

score 2 · Answer 3 · answered Oct 05 '09 at 14:51

2

If you know the exception will always be thrown, why does it matter. Just set the variable to something so it can compile:

void foo(out int x)
{
  if( condition() ) { x = bar(); return; }

  x = 0;

  MyMethodThatAlwaysThrowsAnException( "missed something." );
}

answered Oct 05 '09 at 14:51

Dan Herbert

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That's exactly the thing I wanted to avoid doing. :) – k0dek0mmand0 Oct 05 '09 at 14:53

score 2 · Answer 4 · answered Oct 05 '09 at 14:53

2

There's no way of marking a method in this way.

Possibly irrelevant, but the pattern in your example, using an out parameter, is a bit odd. Why not just have a return type on the method instead?

int Foo()
{
    if (condition()) return bar();

    MyMethodThatAlwaysThrowsAnException("missed something.");
}

answered Oct 05 '09 at 14:53

LukeH

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That was just a simple example. The actual code is rather more complex. – k0dek0mmand0 Oct 05 '09 at 14:57
@k0dek0mmand0: I suspected that might be the case. I guess you're out of luck - there's no way to tell the compiler that `MyMethodThatAlwaysThrowsAnException` always throws. – LukeH Oct 05 '09 at 14:59

score 1 · Answer 5 · answered Oct 05 '09 at 14:52

1

x is an out parameter and must be set before you move forward

answered Oct 05 '09 at 14:52

Chris Ballance

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unless an exception is thrown – k0dek0mmand0 Oct 05 '09 at 14:55
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It is still good policy to ensure that any out parameters are set early on in the method and assign defaults to them if not. – Chris Ballance Oct 05 '09 at 15:00

score 1 · Answer 6 · answered Oct 05 '09 at 16:01

1

If you don't want to have to set x, why don't you just use a ref parameter instead?

void foo(ref int x)
{
  if( condition() ) { x = bar(); return; }

  // nobody complains about anything

  MyMethodThatAlwaysThrowsAnException( "missed something." );
}

answered Oct 05 '09 at 16:01

Maximilian Mayerl

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score 1 · Answer 7 · edited Jan 18 '22 at 13:05

1

To mark method as always throwing you can use [DoesNotReturn] attribute.

Attribute description: https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.doesnotreturnattribute?view=net-6.0

Source: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/nullable-analysis#stop-nullable-analysis-when-called-method-throws

edited Jan 18 '22 at 13:05

Tomerikoo

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answered Jan 18 '22 at 12:49

karolgro

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score 0 · Answer 8 · edited Aug 02 '12 at 20:12

0

It doesn't answer your question but when using out parameters it is always a good idea to initialize them in the beginning of the method. This way you won't have any compiler errors:

void foo(out int x)
{
    x = 0;
    if( condition() ) { x = bar(); return; }
    MyMethodThatAlwaysThrowsAnException( "missed something." );
}

edited Aug 02 '12 at 20:12

Matthew Whited

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answered Oct 05 '09 at 14:52

Darin Dimitrov

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I think out parameters should be assigned where it makes sense or as it may prevent some hidden bugs at compile time just because you forgot to assign out parameter at some code path. – Dzmitry Huba Oct 05 '09 at 14:57

How to mark a method will throw unconditionally?

8 Answers8