elsa-workflows/elsa-core · error · NotSupportedException

Cannot deserialize to .

Error message

Cannot deserialize {value.GetType()} to {typeof(T)}.

What it means

JsonObjectExtensions.GetNullableValue/GetValue-like conversion helper in Elsa.Api.Client throws NotSupportedException when a JSON node value cannot be converted to the requested type T. The helper handles enums, JsonValue nodes, strings, and a few primitives; any other JsonNode subtype (e.g. JsonObject or JsonArray) falls through to this throw.

Solutions

1. Check the JsonNode kind before calling (ValueKind / is JsonObject, is JsonArray) and use JsonSerializer.Deserialize<T> for object/array nodes
2. Change T to a matching type (e.g. Dictionary<string,object>, JsonElement)
3. Wrap in try-catch (NotSupportedException) and provide a default

Example fix

// before
var count = json.GetPropertyOrDefault<int>("customProps");
// after
var props = json.GetPropertyOrDefault<Dictionary<string, object?>>("customProps");

var node = json["prop"];
if (node is not (JsonValue or null)) throw new InvalidOperationException("Expected a scalar JSON value");

static bool IsScalarJsonNode(JsonNode? n) => n is JsonValue or null;

try { value = json.GetPropertyOrDefault<T>(name); } catch (NotSupportedException) { value = default; }

When it happens

Trigger: Calling the extension with T that does not match the underlying JSON value, e.g. the property is a nested object or array (JsonObject/JsonArray) but T is a primitive like int or string.

Common situations: Reading custom properties from workflow definitions where a value is a dictionary or list but the client code tries to read it as a scalar; schema drift between Studio-created properties and client expectations.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Failed to deserialize
• Expected a string tag.
• Expected a string metadata value.
• Invalid variable test values.
• The binding to activity type

AI-assisted analysis of elsa-workflows/elsa-core@fe9217bdfa (2026-09-13). Data as JSON: /api/errors/8fdaf801a187ad06. Report an issue: GitHub.

Appendix: source

Thrown at src/clients/Elsa.Api.Client/Extensions/JsonObjectExtensions.cs:90

        options ??= new() { PropertyNamingPolicy = JsonNamingPolicy.CamelCase };

        if (value is JsonObject jsonObject)
            return JsonSerializer.Deserialize<T>(jsonObject, options)!;

        if (value is JsonArray jsonArray)
            return JsonSerializer.Deserialize<T>(jsonArray, options)!;

        if (typeof(T).IsEnum || (Nullable.GetUnderlyingType(typeof(T))?.IsEnum ?? false))
        {
            if (value.GetValueKind() == JsonValueKind.Null)
                return default!;
            return (T)Enum.Parse(Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T), value.ToString());
        }

        if (value is JsonValue jsonValue)
            return jsonValue.GetValue<T>();

        throw new NotSupportedException($"Cannot deserialize {value.GetType()} to {typeof(T)}.");
    }

    /// <summary>
    /// Sets the property value of the specified model.
    /// </summary>
    /// <param name="model">The model to set the property value on.</param>
    /// <param name="value">The value to set.</param>
    /// <param name="path">The path to the property.</param>
    public static void SetProperty(this JsonObject model, JsonNode? value, params string[] path)
    {
        model = GetPropertyContainer(model, path);
        model[path.Last()] = value?.SerializeToNode();
    }

    /// <summary>
    /// Sets the property value of the specified model.
    /// </summary>
    /// <param name="model">The model to set the property value on.</param>

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