Question: Should non-`ValueType` action parameters _always_ be non-`null`?

[dougbu]

objects are created for unbound POCO parameters only when using ComplexTypeModelBinder. This leads to the following inconsistent behaviours:

• [FromBody] parameters are left null when the incoming request has an empty body. See #4750 for example.
• POCO types with a string converter (i.e. SimpleTypeModelBinder handles them) are left null when the form contains no corresponding data.
• IFormFile parameters are left null when no matching files are submitted.

Empty collections are created in most unbound and IsTopLevelObject cases due to activation code in CollectionModelBinder, HeaderModelBinder et cetera. But byte[] arrays are often left null e.g. when the corresponding <input/> was left blank.

The question is whether we should address these inconsistencies in 1.1.0. Should certainly check how earlier Web API and MVC releases handle the above cases. But might decide these inconsistencies are warts regardless. Or could split the scearios into "API" and "view" groups, then make separate decisions for each.

[Eilon]

What's the actual problem here? What observed behavior is odd, unusual, or difficult to work with?

[Eilon]

The first bullet point is handled by the associated bug.

The second bullet point doesn't seem concerning.

The third bullet point already has the right behavior: if no file is provided, there's no data. That seems to provide the best developer experience.