HttpStatusCode

https://docs.microsoft.com/en-us/dotnet/api/system.net.httpstatuscode?view=netframework-4.7.2

422 UnprocessableEntity

What HTTP status response code should I use if the request is missing a required parameter?

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