dotnetcore/CAP · error · NotSupportedException

Type is not of type JsonElement

Error message

Type is not of type JsonElement

What it means

A type guard in the JsonUtf8 serializer's Deserialize(object value, Type valueType) overload: this implementation can only rehydrate values that arrived as System.Text.Json JsonElement nodes (the shape produced when CAP deserializes the message body with JsonUtf8). Passing a string, byte[], JObject, or any other runtime type reaches the guard and fails with NotSupportedException; the faulty input is the boxed 'value' argument, not the target valueType.

Solutions

1. Convert the input to a JsonElement first (e.g. parse a JSON string with JsonSerializer.Deserialize<JsonElement>(json)) before calling this overload.
2. If you need to deserialize from string or bytes, call the string-based Deserialize(json) overload instead.
3. When substituting serializers in CAP options, ensure producer and consumer both use the same serializer so bodies arrive as JsonElement.
4. Use a custom ISerializer implementation matching your wire format (registered in AddCAP) instead of relying on JsonUtf8Serializer for non-JsonElement inputs.

When it happens

Trigger: Thrown at src/DotNetCore.CAP/Serialization/ISerializer.JsonUtf8.cs:56 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/fcc1502be73aa8af. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP/Serialization/ISerializer.JsonUtf8.cs:56

        return new ValueTask<Message>(new Message(transportMessage.Headers, obj));
    }

    public string Serialize(Message message)
    {
        return JsonSerializer.Serialize(message, _jsonSerializerOptions);
    }

    public Message? Deserialize(string json)
    {
        return JsonSerializer.Deserialize<Message>(json, _jsonSerializerOptions);
    }

    public object? Deserialize(object value, Type valueType)
    {
        if (value is JsonElement jsonElement) return jsonElement.Deserialize(valueType, _jsonSerializerOptions);

        throw new NotSupportedException("Type is not of type JsonElement");
    }

    public bool IsJsonType(object jsonObject)
    {
        return jsonObject is JsonElement;
    }
}

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