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MsgPack016 KeyAttribute-derived attributes are not supported by AOT formatters

When a [MessagePackObject]-attributed type attributes its fields and properties with attributes that derive from KeyAttribute rather than from KeyAttribute itself, that type is incompatible with AOT source generated formatters.

Examples of patterns that are flagged by this analyzer

[MessagePackObject]
public class A
{
    [CompositeKey(0, 1)]
    public string Prop1 { get; set; }

    [CompositeKey(0, 2)]
    public string Prop2 { get; set; }
}

public class CompositeKeyAttribute : KeyAttribute
{
    public CompositeKeyAttribute(byte level, int index)
        : base(CreateKey(level, index)) { }

    private static string CreateKey(byte level, int index)
    {
        var c = (char)('A' + level);
        return c + index.ToString("x");
    }
}

Typical fix

If you intend to keep using the derived attributes type, suppress the warning by turning off source generated formatter generation for that type:

-[MessagePackObject]
+[MessagePackObject(SuppressSourceGeneration = true)]
 public class A

Alternatively, enable propery AOT formatter source generation by switching to the standard KeyAttribute:

[MessagePackObject]
public class A
{
    [Key("A1")]
    public string Prop1 { get; set; }

    [Key("A2")]
    public string Prop2 { get; set; }
}