feature: Improve support for nullable

[dannyBies]

Is your feature request related to a problem? Please describe.

In my project I have nullable reference types enabled <Nullable>enable</Nullable>.

I retrieve data like this:

var response = await RestApi.Users.Query.GetUrl("1234");
if (!response.IsSuccessStatusCode)
{
    Logger.LogError(response.Error.Message);
    return;
}
var url = response.Content.Url;

The issue I'm seeing is that I get CS8602 warnings(Dereference of a possible null reference) for response.Content and response.Error due to these being nullable. However I am checking response.IsSuccessStatusCode which I believe make sure that either Content or Error is populated.

Describe the solution you'd like

To not get these warnings.

Describe suggestions on how to achieve the feature

I believe we can add the following attributes to IsSuccessStatusCode to fix this issue.
[MemberNotNullWhen(true, nameof(Content))]
[MemberNotNullWhen(false, nameof(Error))]

[clairernovotny]

Can you please share a sample interface definition that matches what you're doing? It's not clear what the type of response is here.

[dannyBies]

This happens for every call that uses ApiResponse

refit/Refit/ApiResponse.cs

Line 45 in 569108a

public T? Content { get; }

An example could be:

[Get("/api/xyz")]
Task<ApiResponse<UrlResponse> GetUrl(CancellationToken cancellationToken = default);

public record UrlResponse(string Url);

[clairernovotny]

Can you please submit a PR to add the correct attributes?

[dannyBies]

I've submitted a PR, thanks!

[clairernovotny]

Are there other places in the public API that we need to decorate with the nullable attributes to be more accurate?

[dannyBies]

I've looked through ApiResponse and ApiException and have added one more property to the list. I haven't looked through the rest of the API as I'm not too familiar (yet!) with the project and I unfortunately don't have the time right now to look through everything.

If you can point me to the exact properties that could also use this change I'd be happy to make another commit.

[clairernovotny]

Thanks!

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