Query String ?& empty parameter Web API

Question

Having the following action:

public IHttpActionResult GetStuff(
    string like = null,
    [FromUri]string[] state = null,
    [FromUri]string[] mode = null,
    [FromUri]string[] label = null,
)

when I query it as /api/stuff?state=A&state=B the model binder instantiates state array with 2 string values and it's kind enough to instantiate empty mode and label arrays so I don't need to check for nulls. However, if I query it as /api/stuff?&state=A&state=B (note extra ampersand) mode and label arrays are no longer empty - they contain a single null elements both. Why?

In my understanding the query strings are equivalent. Is there any way to fix it without writing custom binder?

Jamiec · Accepted Answer · 2014-12-04 13:48:47Z

3

The two query strings are not equivalent. Your second query has a second, nameless, parameter.

/api/stuff?state=A&state=B = query-string with 1 parameter, state.

/api/stuff?&state=A&state=B = query-string with 2 parameters, state and a nameless one.

?& translates to a nameless parameter name having a null value.

The Route parser correctly parses state. However it detects a second parameter and it doesn't know where to put it. It ends up putting it in both your other parameters.

The Route parser's logic is similar with this:
- I'm looking for state ... oh here is it, let's populate.
- I'm looking for mode ... nothing found but hey there's a nameless one, let's populate with it.
- I'm looking for label ... nothing found but hey there's a nameless one, let's populate with it.

PS: you can send a nameless query-string parameter like this:

http://www.myurl.com/?=value

edited Dec 4, 2014 at 13:48

Jamiec

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answered Dec 4, 2014 at 13:20

Mihai Dinculescu

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4 Comments

Jamiec Over a year ago

use backticks for code, for word emphasis you can use italics or bold (Ive edited your answer)

Mihai Dinculescu Over a year ago

Thank you for the tip, I will sure do so from now on.

UserControl Over a year ago

OMG, this is a weirdest behavior one could expect. In case if, say, there is &extra=1 parameter in the query string the binder still doesn't know where to put 1 (and it doesn't). In other words, if there are no matches there is no reason to put non-matching values anywhere.

Mihai Dinculescu Over a year ago

In your example if wouldn't put the 1 somewhere else. The situation I have described above happens only with nameless parameters.

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