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

问题

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?

回答1:

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)

回答2:

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

回答3:

result = null;

回答4:

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

回答5:

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.

回答6:

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

来源：https://stackoverflow.com/questions/862207/what-should-the-out-value-be-set-to-with-an-unsuccessfull-tryxx-method

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tryparse