WithEnforceNullabilty() has unpredictable behavior with classes having both nullable reference type and value type

[popemkt]

Describe the bug

When deserializing to a class with both nullable reference and value types members, YamlDotNet considers the nullabe value type to not accept null values.

I suspect it's coming from the reflection checks only checking if a member allows null using the presence of NullableAttribute, which fails if it's a TValueType? (this won't generate a NullableAttribute). But I've seen strange cases where the compiler doesn't even generate NullableAttribute on a reference type, so I won't be too sure about that.

To Reproduce
Using latest version: 16.2.1

Create a new console project with .net8
Program.cs

#nullable enable
using YamlDotNet.Serialization;

var deserializer = new DeserializerBuilder().WithEnforceNullability().Build();
var yaml = @"
TestString: null
TestBool: null
";
var test = deserializer.Deserialize<NonNullableClass>(yaml);

public class NonNullableClass
{
    public string? TestString { get; set; }
    public bool? TestBool { get; set; }
}

Result:

Unhandled exception. (Line: 2, Col: 1, Idx: 2) - (Line: 2, Col: 11, Idx: 12): Strict nullability enforcement error.

[popemkt]

Update, the example will give an error even when I use the exact setup as in one of the tests, but with a ? notation on the property

#nullable enable
using YamlDotNet.Serialization;

var deserializer = new DeserializerBuilder().WithEnforceNullability().Build();
var yaml = @"
TestString: null
";
var test = deserializer.Deserialize<NonNullableClass>(yaml);

public class NonNullableClass
{
    public string? TestString { get; set; }
}

Result: Unhandled exception. (Line: 2, Col: 1, Idx: 2) - (Line: 2, Col: 11, Idx: 12): Strict nullability enforcement error.
Here's the lowered version of the class (no NullableAttribute on the property)

[NullableContext(2)]
[Nullable(0)]
public class NonNullableClass
{
  [CompilerGenerated]
  [DebuggerBrowsable(DebuggerBrowsableState.Never)]
  private string <TestString>k__BackingField;

  public string TestString
  {
    [CompilerGenerated] get
    {
      return this.<TestString>k__BackingField;
    }
    [CompilerGenerated] set
    {
      this.<TestString>k__BackingField = value;
    }
  }

  public NonNullableClass()
  {
    base..ctor();
  }
}

I think this is because way the compiler generate NullableAttribute is really unreliable to base the checks on.

[EdwardCooke]

I’m not at a computer for a bit. But. In your second example, what happens when you put a non nullable, no question mark, string in there? What is generated? I wonder if there’s an optimization where if all properties/members are nullable then it only puts the nullable attributes at the class level and not on individual properties and fields. That’s just a guess though.

[popemkt]

I guess you might be right,
Here's the version without ?

[NullableContext(1)]
[Nullable(0)]
public class NonNullableClass
{
  [CompilerGenerated]
  [DebuggerBrowsable(DebuggerBrowsableState.Never)]
  private string <TestString>k__BackingField;

  public string TestString
  {
    [CompilerGenerated] get
    {
      return this.<TestString>k__BackingField;
    }
    [CompilerGenerated] set
    {
      this.<TestString>k__BackingField = value;
    }
  }

  public NonNullableClass()
  {
    this.<TestString>k__BackingField = "Default";
    base..ctor();
  }
}

[popemkt]

@EdwardCooke did you have any time to look at this?

[EdwardCooke]

Thanks for the reminder. I just did some digging. Looks like there is some optimization that happens with nullable properties at the compiler level.

dotnet/roslyn#39133

I have implemented the required changes so we should be good once I cut a release for it.