map empty query strings to nullable types

[mnbob70]

I created a GET search minimal API that included optional startdate and end date parameters. I set the parameter types to nullable datetime. I incorrectly expected an empty query string to map to null, but instead it generated 400 Bad Request errors.

This makes it impossible to process an HTML form using GET without processing all of the parameters as strings, performing null checks and finally converting to the expected type. Is there a reason this can't be handled with nullable parameter types?

[engineering87]

Hello @mnbob70,
the answer is yes, that’s expected behavior.
In Minimal APIs a DateTime? becomes null when the query parameter is missing. If the parameter is present but empty (?startDate=), the binder still tries to parse the empty string as a DateTime. The parse fails and the framework returns 400 Bad Request.
So, the key difference is between a missing value and an invalid value. An empty query parameter is treated as an invalid value, not as null.
In practice people usually solve it by omitting empty parameters from the query string, binding as string? and parsing manually, or normalizing empty values with a custom binder (BindAsync).

[mnbob70]

A similar issue was fixed for POST but it does not seem to have been done for GET.
#52195
#52499
#60498

With Blazor SSR it should be possible to process a Form GET with model binding. I was trying to use Blazor with HTMX and Form GET instead of POST.