This bug vaguely follows from investigating into https://aspnetwebstack.codeplex.com/workitem/1493. When multiple request values for the same parameter are sent by the client, the model binder attempts to convert the entire collection to the action parameter value instead of picking the first request value.
Repro scenario
public IHttpActionResult Get([FromUri]string foo)
For the query string ?foo=bar&foo=qux
Expected:
foo = "bar"
Actual:
foo = "(Collection)"
Repro scenario
public IHttpActionResult Get([FromUri]string foo)
For the query string ?foo=bar&foo=qux
Expected:
foo = "bar"
Actual:
foo = "(Collection)"