[Discuss]: How do we handle optional flattened property?

[archerzz]

Problem

This is derived from comment here. Consider the following scenario of property flattening.

model Foo {
  name: string;

  @flattenProperty
  properties?: B; // optional property to be flattened
}

model Bar {
  a: int; // required
  b?: int; // optional
}

In M4, both Bar.a and Bar.b will be flattened into Foo and will all be optional. That is, the expected CSharp model codes should be like.

public partial class Foo
{
    public string Name { get; set; }
    public int? A { get; set; }
    public int? B { get; set; }
}

However, that will have a few problems.

1. If we change A to be optional, then our serialization logic doesn't check if both A and B are null, before writing value of properties. We should skip properties if both A and B are null.
2. If we change A to be optional, how do we enforce it must be set if any other property on the same level is set? Like, a customer sets foo.B = 12, but leaves foo.A to be null.
   a. Check A during serialization, which will surprise users since A is int?.
   b. Don't check, wait for the server side to return error, which will still surprise users.
3. If we choose to maintain the original optionality, then how do we give users a way to unset properties?

[archerzz]

close since this issue has existed for quite a long time, and we only allow model flattening in brown field SDKs.