how to have conditional params arguments to a function

Question

I have

public void Foo(string name, params object[] args)

I want to call it with a list of args that can vary. I need to do something like

Foo("yo",a,b,x==42?c);

ie if x is 42 then pass in a,b,c otherwise pass in a,b. Of course that syntax doesn't work. I know I could marshal the args to a List and pass the list to the function, but that would be very messy to do the way the code is organized. So is there some syntax magic i can use

EDIT: Let me add the concrete case

var xml = new XDocument(....,
   new XElement(....),
   new XElement(....),
   new XElement(....),
   x==42? new XElement(.....),
   new XElement(....),
   new XElement(....)
   ....

Dave Cousineau · Accepted Answer · 2013-05-21 21:13:39Z

11

You can just use an if statement:

if (x == 42)
   Foo("yo", a, b, c);
else
   Foo("yo", a, b);

You can't use the ?: operator in this case (at least, outside of the function call), because Foo has no return value. The ?: operator must evaluate to something, and that something must be assigned to something else.

Another option that would get rid of the duplicate function call is to use an array or list for the params:

var parameters = new List<object> { a, b };

if (x == 42)
   parameters.Add(c);

Foo("yo", parameters);

And if you really wanted to get ?: in there, this would work, too:

Foo("yo", x == 42 ? new object[] { a, b, c } : new object[] { a, b });

For your more specific question about the XDocument/XElement constructor calls, you might want to use Add calls rather than one long series of constructor calls. Then you can make them conditional. As is, I think you should also be able to do what you're asking by doing something like this:

XElement elementPrecedingOptionalElement = new XElement(...);

var xml = new XDocument(....,
   new XElement(...),
   new XElement(...),
   elementPrecedingOptionalElement,
   new XElement(...),
   new XElement(...)
);

if (x == 42)
   elementPrecedingOptionalElement.AddAfterSelf(new XElement(...));

Using Add calls would look something like this:

XDocument xml = new XDocument();
XElement root = new XElement("Root");
xml.Add(root);

root.Add(new XElement("Item1"));
root.Add(new XElement("Item2"));
if (x == 42)
   root.Add(new XElement("Item2.5"));
root.Add(new XElement("Item3"));
root.Add(new XElement("Item4"));

Actually, one last version that is much closer to what you are asking for would be like this, which seems to work:

var xml = new XDocument(....,
   new XElement(...),
   new XElement(...),
   new XElement(...),
   x == 42
      ? new XElement(...)
      : null,
   new XElement(...),
   new XElement(...)
);

edited May 21, 2013 at 21:13

answered May 21, 2013 at 19:00

Dave Cousineau

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3 Comments

Tim S. Over a year ago

+1 for List<> option: it's the closest to what the question needs (even if it'll be "messy" as he said).

pm100 Over a year ago

see my edit to see why this doesnt work for me. PS I know how to make it work, I am trying to see if I can make it work easily and clearly - in particular I want to make the intent clear. I dont wnat to use ?:, it was just my first attempt (that I immediately gave up on)

pm100 Over a year ago

@Sahuagin - ty - you did what i was too lazy to do - see what happens if you pass in null as an xelement, I just assumed that Bad Things (tm) would happen. And the answer is that it works just fine

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