Create custom metadata field in Azure Digital Twins

Question

I would like to create a custom Metadata field $frq, similar to the $model field for my digital twins. I can create a CustomDigitalTwin class in C#, there is no error when trying to create the twin with the DigitalTwins.Core SDK. However, it doesn't show in Azure Digital Twins Explorer and I also cannot retrieve my custom metadata field FRQ with C#.

Here is my Code and my classes:

string twinId = "test-twin";
var initData = new TestDigitalTwin
{
   Id = twinId,
   Metadata = 
   {ModelId = "dtmi:com:dtwin:test;1",
    FRQ = "60"},
};


await client.CreateOrReplaceDigitalTwinAsync<CustomDigitalTwin>(twinId, initData);

Response<CustomDigitalTwin> getCustomDtResponse = await client.GetDigitalTwinAsync<CustomDigitalTwin>(twinId);
CustomDigitalTwin customDt = getCustomDtResponse.Value;
Console.WriteLine(customDt.Metadata.FRQ); // FRQ is null here

Classes:

// Define a custom model type for the twin to be created

    internal class TestDigitalTwin
    {
        [JsonPropertyName(DigitalTwinsJsonPropertyNames.DigitalTwinId)]
        public string Id { get; set; }

        [JsonPropertyName(DigitalTwinsJsonPropertyNames.DigitalTwinETag)]
        public string ETag { get; set; }

        [JsonPropertyName(DigitalTwinsJsonPropertyNames.DigitalTwinMetadata)]
        public MyCustomDigitalTwinMetadata Metadata { get; set; } = new MyCustomDigitalTwinMetadata();
    }

    internal class MyCustomDigitalTwinMetadata
    {
        [JsonPropertyName(DigitalTwinsJsonPropertyNames.MetadataModel)]
        public string ModelId { get; set; }

        [JsonPropertyName(DigitalTwinsJsonPropertyNames.MetadataFRQ)]
        public string FRQ { get; set; }

        [JsonPropertyName("temperature")]
        public DigitalTwinPropertyMetadata Temperature { get; set; }

        [JsonPropertyName("humidity")]
        public DigitalTwinPropertyMetadata Humidity { get; set; }
    }
    /// <summary>
    /// String constants for use in JSON de/serialization for custom types.
    /// </summary>
    public static class DigitalTwinsJsonPropertyNames
    {
        /// <summary>
        /// The JSON property name for the Id field on a digital twin.
        /// </summary>
        public const string DigitalTwinId = "$dtId";

        /// <summary>
        /// The JSON property name for the ETag field on a digital twin.
        /// </summary>
        public const string DigitalTwinETag = "$etag";

        /// <summary>
        /// The JSON property name for the metadata field on a digital twin or a component.
        /// </summary>
        public const string DigitalTwinMetadata = "$metadata";

        /// <summary>
        /// The JSON property name for the model field on a digital twin metadata.
        /// </summary>
        public const string MetadataModel = "$model";

        /// <summary>
        /// The JSON property name for the frequency field on a digital twin metadata.
        /// </summary>
        public const string MetadataFRQ = "$frq";

        /// <summary>
        /// The last update time of a digital twin property, used in the $metadata object
        /// on a digital twin or component about their properties.
        /// </summary>
        public const string MetadataLastUpdateTime = "$lastUpdateTime";

        /// <summary>
        /// The last update time of a digital twin property, used in the $metadata object
        /// on a digital twin or component about their properties.
        /// </summary>
        public const string MetadataPropertyLastUpdateTime = "lastUpdateTime";

        /// <summary>
        /// The time the value of a digital twin property was sourced, used in the $metadata
        /// object on a digital twin or component about their properties.
        /// </summary>
        public const string MetadataPropertySourceTime = "sourceTime";

        /// <summary>
        /// The JSON property name for the Id field on a relationship.
        /// </summary>
        public const string RelationshipId = "$relationshipId";

        /// <summary>
        /// The JSON property name for the source Id field on a relationship.
        /// </summary>
        public const string RelationshipSourceId = "$sourceId";

        /// <summary>
        /// The JSON property name for the target Id field on a relationship.
        /// </summary>
        public const string RelationshipTargetId = "$targetId";

        /// <summary>
        /// The JSON property name for the name field on a relationship.
        /// </summary>
        public const string RelationshipName = "$relationshipName";
    }

    public class DigitalTwinPropertyMetadata
    {
        /// <summary>
        /// The date and time the property was last updated.
        /// </summary>
        [JsonPropertyName(DigitalTwinsJsonPropertyNames.MetadataPropertyLastUpdateTime)]
        public DateTimeOffset LastUpdatedOn { get; set; }

        /// <summary>
        /// The date and time the value of the property was sourced.
        /// </summary>
        [JsonPropertyName(DigitalTwinsJsonPropertyNames.MetadataPropertySourceTime)]
        public DateTimeOffset? SourceTime { get; set; }
    }

Answer 1

The API does not support adding custom metadata fields. You can update the model, or update the sourceTime property in the $metadata object, but adding a custom field is not supported. But the API is pretty inconsistent in letting the user know. For instance, when you create a new twin with the following body (I'm doing this against the REST API directly with Postman)

{
  "$metadata": {
    "$model": "dtmi:some:model;2",
    "$stuff": "foo"
  },
  "someProperty" : "bar"
}

The result is 200 OK with the response:

{
    "$dtId": "the-id",
    "$etag": "W/\"908d9907-971e-4036-b269-d0595d6e04b7\"",
    "someProperty": "bar",
    "$metadata": {
        "$model": "dtmi:some:model;2",
        "someProperty": {
            "lastUpdateTime": "2022-07-25T11:33:15.2115802Z"
        }
    }
}

You can see the custom property vanished, but the API says it's okay. However , when I try adding a custom property through a PATCH, this is what happens. Request:

[
  {
    "op": "add",
    "path": "/$metadata/$foo",
    "value": "stuff"
  }
]

The result is 400 Bad Request

{
    "error": {
        "code": "JsonPatchInvalid",
        "message": "Cannot patch path /$metadata/$stuff Please provide a valid patch document. See section on update apis in the documentation https://aka.ms/adtv2twins."
    }
}