Results.Json(null) stopped producing a response body in .NET 8 (and 7)

[WGroenestein]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

In .NET 8 (and 7) when a null value is given to Results.Json, it no longer outputs "null" in the response body, instead the response body is empty

Expected Behavior

null

is put in the response body

Steps To Reproduce

https://github.com/WGroenestein/ResultJsonRepro

Exceptions (if any)

No response

.NET Version

8.0.100

Anything else?

Since .NET 8 (the new LTS) was released Nov 2023 and .NET 6 (current targeted LTS) is EOL Nov 2024 we are trying to switch our projects over to .NET 8 (LTS => LTS upgrade). However this LTS upgrade causes our API contracts to break.

We didn't find anything related to this breaking change in the msdn docs (https://learn.microsoft.com/en-us/dotnet/core/compatibility/7.0 and https://learn.microsoft.com/en-us/dotnet/core/compatibility/8.0)

[teociaps]

The issue you're encountering seems to be relative to a change in JSON result handling between .NET versions 6 and 7/8, particularly in the context of minimal APIs. In .NET 6, the JsonResult class from MVC was used, whereas in newer versions 7 and 8, a new typed result class called JsonHttpResult<TValue> is utilized behind the scenes.

The key distinction lies in how null values are managed. With JsonHttpResult<TValue>, if the Value passed to it is null (your case), the JsonHttpResult<TValue>.ExecuteResultAsync method returns before writing the JSON result to the response body:

and then, if Value is not null:

JsonResult, on the other hand, behaves differently. It sets a default content type (application/json) and serializes the value to the response body regardless of whether it's null or not.

This is why Results.Json(null) behaves as you expected only in MVC/API Controller in .NET 7/8, while in minimal API it doesn't. In contrast, in .NET 6, it works consistently across both scenarios.

[captainsafia]

@teociaps Thanks for taking the time to post this explainer.

It doesn't look like we documented this change in our announcements repo at https://github.com/aspnet/Announcements. I'll go ahead and file an issue to document this.

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[captainsafia]

Still on my radar to add the breaking change notice here.