Why are the return types of .NET's System.Text.Json.JsonSerializer.Deserialize methods nullable when it doesn't return null?

Question

Consider .NET 6's System.Text.Json.JsonSerializer.Deserialize method and its many overloads.

Each one returns either object? or TValue? which indicates that null could be returned. However, the documentation states that in the event of any error, an exception is thrown. Moreover, it also states that a valid value is returned (if an exception isn't thrown). Nowhere does it state that null could be returned. And in my experience of using it, null is never returned.

(The nullable return type is a problem if you turn on "nullability" in your project, because the compiler then starts warning you that the returned value could be null - which in my experience (and according to the documentation) just isn't true. It's this sort of thing that has me scurrying to switch nullability off again. I know I can suppress the warning, but I don't think that's the point.)

Is the documentation lacking? Can the methods really return null? If they can't, then why do we have nullable return types?

Virtue

null is valid JSON. – Fairfax

Answer 1

• TValue? Deserialize<TValue> is just a wrapper for Object? Deserialize(type: typeof(TValue)).
• When #nullable annotations were added to C# 8.0, the System.Type type was not extended with support for those annotations.
  • Therefore, it is not possible for a generic-type in C# to know (at runtime) if a generic-type-parameter's concrete type-argument is a nullable-reference-type or a non-nullable-reference-type.
    • _{(This doesn't apply to Nullable<T>, of course)}
  • UPDATE: In .NET 6 it is now possible to use runtime reflection to inspect nullable annotations with NullabilityInfoContext.
• So, at runtime, a call-site for Deserialize<TValue>() using either TValue := MyClass? or TValue := MyClass will behave identically because the Deserialize method has no idea if MyClass has the ? annotation or not.
  • The Deserialize<TValue> method only knows if TValue is a reference-type or not (via typeof(TValue)).
• Therefore, if you instruct JsonSerializer to deserialize the string "null" (which is valid JSON) and if TValue is a reference type, then it will return null.
  • ...even if your TValue doesn't have ?. Remember that JsonSerializer doesn't (and cannot) know that!
• Which is why the method's static return-type has the ? annotation, because if it didn't then it would be incorrectly asserting it never returns null.

---

It can be argued that JsonSerialize's designers could have added some more methods that allowed callers to statically assert non-null-ness, but they didn't, but you can if you really want to:

Like so:

public static TValue DeserializeNonNullObject<TValue>( String json )
    where TValue : class
{
    TValue? orNull = JsonSerializer.Deserialize<TValue>( json );
    return orNull ?? throw new JsonException( "Expected deserialized object to be non-null, but encountered null." );
}

Fairfax

Thanks so much. This has helped me understand many things. I did not know - or forgot - that "null" was valid JSON. – Virtue

Answer 2

According to RFC 7159 “null” (and “true” and “false”) will be valid JSON text. However, empty string is not a valid JSON.

Coquetry

Thanks! And thanks to @Dai! I get it now. And I tested it out and confirmed the result: var x = JsonSerializer.Deserialize<Test>("null"); // returns null. Finally I understand. So maybe I really should switch on "nullability" and pay attention to the warnings! (Up until now I always assumed that null just wouldn't be returned. I was wrong.) – Virtue