System.Text.Json serializing null becomes "null" as 'serialized' object

[Fazer01]

Hi there,

Perhaps I'm mistaken but when serialize null as in the test below, I'd expect the JsonSerializer (System.Text.Json for .NET8.0) to throw an exception of some sort, but it doesn't. What is the rationale behind this implementation, or where to find the specific documentation about this behaviour?

Simple test:

public class SimpleTests
{
    [Fact]
    public void GivenInvalidObject_WhenSerializeNull_ShouldThrowException()
    {
        Action act = () => JsonSerializer.Serialize<TestClass>(null);

        act.Should().Throw<Exception>();
    }

    [Fact]
    public void GivenInvalidObject_WhenSerializeNull_ShouldBeNullOrEmpty()
    {
        string s = JsonSerializer.Serialize<TestClass>(null);
        //s = "null"
        s.Should().BeNullOrEmpty();
    }
}

TestClass

public class TestClass
{
    public string Foo { get; set; }
    public int Bar { get; set; }
}

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[Fazer01]

@forens7

[huoyaoyuan]

The behavior is correct. null is a valid top-level JSON value. Trying in browser console you will get:

> JSON.stringify(null)
'null'
> JSON.parse('null')
null

That's why the return value of JsonSerializer.Parse<T> is always nullable, since there's top-level "null" literal. See also #100144 (comment)

[Fazer01]

Thanks for replying so quickly and elaborate on this, and will continue reading in #100144