Status 422 seems most appropiate based on the spec.

The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.

They state that malformed xml is an example of bad syntax (calling for a 400). A malformed query string seems analogous to this, so 400 doesn't seem appropriate for a well-formed query-string which is missing a param.

UPDATE @DavidV correctly points out that this spec is for WebDAV, not core HTTP. But some popular non-WebDAV APIs are using 422 anyway, for lack of a better status code (see this).

system.net.HttpStatusCode  没有UnprocessableEntity枚举类型，422。

https://github.com/dotnet/corefx/issues/4382

https://github.com/dotnet/core/issues/1335    但是.net core 2.1已经支持了

304

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304

The HTTP 304 Not Modified client redirection response code indicates that there is no need to retransmit the requested resources. It is an implicit redirection to a cached resource. This happens when the request method is safe, like a GET or a HEAD request, or when the request is conditional and uses a If-None-Match or a If-Modified-Since header.

The equivalent 200 OK response would have included the headers Cache-Control, Content-Location, Date, ETag, Expires, and Vary.