JsonSerializerOptions.PropertyNamingPolicy not effect ExpandoObject

[ahdung]

dynamic foo = new ExpandoObject();
foo.Bar = 3;
JsonSerializer.Serialize(foo, new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });
//result: {"Bar":3}

[ghost]

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Issue Details

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dynamic foo = new ExpandoObject();
foo.Bar = 3;
JsonSerializer.Serialize(foo, new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });
//result: {"Bar":3}

Author:	ahdung
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eiriktsarpalis]

I would expect JsonNamingPolicy to only concern matching json property names to .NET member names. ExpandoObject is something different but I can see why users might expect it to have similar behaviour. @layomia is this something we could consider fixing?

[layomia]

This is being addressed along with the work on dynamic - #29690, dotnet/designs#163. I believe the current thinking is to honor this and related options when the feature lands. cc @steveharter.

[eiriktsarpalis]

Triage:

• We are working on bringing dynamic support via Support "dynamic" and writable DOM #29690, this will not use ExpandoObject.
• It might still make sense to add this improvement for ExpandoObject in the future.
• Need to check: how does Newtonsoft treat ExpandoObject serialization with camelCase settings?

[dbc2]

In .NET 5, System.Text.Json serializes ExpandoObject as an IDictionary<string, object> so setting JsonSerializerOptions.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase will cause expandos to be serialized using camel casing.

Demo here: https://dotnetfiddle.net/jf29xe.