What should the out value be set to with an unsuccessfull TryXX() method?

Question

I'm implementing a TryParse(string s, Out object result) method. If the parse fails, I would like not to touch the out parameter so any previous result will remain intact. But VS2k8 won't let me. I have to set the value of the out object no matter what.

Should I just put result = result for the sake of pleasing the compiler? Am I missing something?

Answer 1

result = null;

Answer 2

Assign null (or default(T) more generally). You must assign a value, that's what 'out' means.

Answer 3

Just put some default value. For example the Int32.TryParse method puts zero.

Answer 4

Your suggestion of result = result won't work, because it's an out parameter - it's not definitely assigned to start with, so you can't read its value until you've assigned a value to it.

result = null;

is definitely the right way to go for an object out parameter. Basically use default(T) for whatever type T you've got. (The default operator is useful in generic methods - for non-generic code I'd normally just use null, 0, whatever.)

EDIT: Based on the comment from Boris, it may be worth elaborating on the difference between a ref parameter and an out parameter:

Out parameters

• Don't have to be definitely assigned by the caller
• Are treated as "not definitely assigned" at the start of the method (you can't read the value without assigning it first, just like a local variable)
• Have to be definitely assigned (by the method) before the method terminates normally (i.e. before it returns; it can throw an exception without assigning a value to the parameter)

Ref parameters

• Do have to be definitely assigned by the caller
• Are treated as "definitely assigned" at the start of the method (so you can read the value without assigning it first)
• Don't have to be assigned to within the method (i.e. you can leave the parameter with its original value)

Answer 5

You could throw an exception before the code that is supposed to set result.

Answer 6

You could use ref instead of out if you don't want to assign a value, although this must then be initialised by the caller.