elsa-workflows/elsa-core · error · JsonException

Cannot convert to bool?

Error message

Cannot convert {reader.TokenType} to bool?

What it means

NullableBooleanConverter converts JSON booleans, parseable "true"/"false" strings, and JSON null into bool?. Any other token type (number, object, array, unparseable string) causes Read to throw this JsonException naming the token type.

Solutions

1. Send true, false, null, or the strings "true"/"false".
2. If the source emits 1/0, extend or replace the converter to map numbers to booleans.
3. If the source emits "yes"/"no", pre-map the payload or use a custom converter with a lookup.
4. Check the failing property name via the serializer path in the exception to fix the right field.

Example fix

// before
{"archived": 0}
// after
{"archived": false}

var ok = node.ValueKind is JsonValueKind.True or JsonValueKind.False or JsonValueKind.Null
    || (node.ValueKind == JsonValueKind.String && bool.TryParse(node.GetString(), out _));
if (!ok) throw new FormatException($"Expected bool?/null, got {node.ValueKind}");

bool IsNullableJsonBool(JsonElement e) => e.ValueKind is JsonValueKind.Null or JsonValueKind.True or JsonValueKind.False || (e.ValueKind == JsonValueKind.String && bool.TryParse(e.GetString(), out _));

try { flag = JsonSerializer.Deserialize<bool?>(json, options); }
catch (JsonException ex) { logger.LogWarning(ex, "Skipping invalid nullable boolean"); flag = null; }

When it happens

Trigger: Deserializing a bool? property from a JSON number (1), a non-boolean string ("yes"), an object, or an array; JSON null is fine and yields null.

Common situations: Numeric flag encodings from legacy APIs; localized boolean spellings; accidentally mapping an object property to a bool? field in a model.

Understand the failure class

Background: "failed to unmarshal" / json.Unmarshal errors: why parsing a response into a Go struct fails and how to fix it — this error's family across 23 libraries.

Related errors

• Cannot convert to bool
• Cannot deserialize to .
• Expected a metadata property name.
• Expected a string metadata value.
• Expected a string tag.

AI-assisted analysis of elsa-workflows/elsa-core@fe9217bdfa (2026-09-13). Data as JSON: /api/errors/d0d58260e2b27a52. Report an issue: GitHub.

Appendix: source

Thrown at src/modules/Elsa.Common/Converters/NullableBooleanConverter.cs:24

public class NullableBooleanConverter : JsonConverter<bool?>
{
    public override bool? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        switch (reader.TokenType)
        {
            case JsonTokenType.True:
                return true;
            case JsonTokenType.False:
                return false;
            case JsonTokenType.String:
                var value = reader.GetString();
                if (bool.TryParse(value, out var b))
                    return b;
                break;
            case JsonTokenType.Null:
                return null;
        }
        throw new JsonException($"Cannot convert {reader.TokenType} to bool?");
    }

    public override void Write(Utf8JsonWriter writer, bool? value, JsonSerializerOptions options)
    {
        if (value.HasValue)
            writer.WriteBooleanValue(value.Value);
        else
            writer.WriteNullValue();
    }
}

View on GitHub (pinned to fe9217bdfa)