Minimal API string response - ArgumentNullException

[RichardD2]

According to Create responses in Minimal API applications, a minimal API endpoint can return a string, Task<string>, or ValueTask<string>.

However, this doesn't seem to work if the returned string is null. Using .NET 10.0, given:

app.MapGet("returnString", () => (string?)null);

a request to the endpoint returns:

{
    "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
    "title": "Value cannot be null. (Parameter 'text')",
    "status": 400,
    "detail": "   at System.ArgumentNullException.Throw(String paramName)\r\n
   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)\r\n
   at Microsoft.AspNetCore.Http.HttpResponseWritingExtensions.WriteAsync(HttpResponse response, String text, CancellationToken cancellationToken)\r\n
   ...",
    "instance": "GET /returnString",
    ...
}

Stack trace:

at System.ArgumentNullException.Throw(String paramName)
at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
at Microsoft.AspNetCore.Http.HttpResponseWritingExtensions.WriteAsync(HttpResponse response, String text, CancellationToken cancellationToken)
...

Is this expected behaviour, or is it a bug?

And if it's expected, shouldn't the docs be updated to explicitly call this out?

[martincostello]

I would say this is a bug and a misuse.

It's a bug because the framework shouldn't be trying to write null to the response, it should probably just write nothing. Otherwise it hits the guard here that the method needs the text to have a value:

aspnetcore/src/Http/Http.Abstractions/src/Extensions/HttpResponseWritingExtensions.cs

Line 29 in 552c986

ArgumentNullException.ThrowIfNull(text);

I would say it's a misuse because there's no way to write null to a raw text/plain HTTP response. null would just semantically be the literal characters n u l l because the response is just text.

I'm not sure what the fix for the bug would be - either to throw an explicit exception for the misuse, or to just no-op and write nothing.

[RichardD2]

I'm torn between wanting it to not write anything, and wanting it to return 204: No Content instead. :)

[RichardD2]

For now, I've worked around the issue by changing the endpoint to return Results<Ok<string>, NoContent> instead of just string.

app.MapGet("returnString", () => valueToReturn is null 
    ? TypedResults.NoContent() 
    : TypedResults.Ok(valueToReturn));