microsoft/aspire · error · JsonException

Expected PropertyName, got

Error message

Expected PropertyName, got {reader.TokenType}

What it means

While iterating the members of a dictionary-shaped JSON object, the converter expects each token after StartObject (or after a value) to be a PropertyName; any other token throws JsonException. This indicates malformed or structurally unexpected JSON rather than a value-level problem.

Solutions

1. Validate the JSON file with a JSON linter/parser to find the structural error near the reported property.
2. Repair the object so every entry is "name": value pairs inside the braces.
3. If generated, fix the generator to emit a plain JSON object for this property.

Example fix

// before (array where object properties expected)
"featureFlags": ["A", "B"]
// after
"featureFlags": { "A": true, "B": false }

try { using var _ = JsonDocument.Parse(json); } // full structural validation first
catch (JsonException ex) { throw new FormatException($"Malformed JSON: {ex.Message}"); }

try
{
    var config = JsonSerializer.Deserialize<MyConfig>(json, options);
}
catch (JsonException ex) when (ex.Message.StartsWith("Expected PropertyName"))
{
    logger.LogError(ex, "Malformed JSON object structure in configuration.");
}

When it happens

Trigger: Malformed JSON where an array or scalar appears where a property name should be; JSON that closes the object then contains trailing tokens reaching the converter loop; programmatically constructed reader sequences.

Common situations: Hand-crafted or machine-generated JSON with structural mistakes; interleaved edits corrupting a config file; a serializer upstream writing an unexpected shape for this section.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

• Cannot convert string
• Error deserializing GenAI message content. Error description
• Expected a non-empty Kubernetes MicroTime value.
• Expected a string token for Kubernetes MicroTime but found
• Expected StartObject, got

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/166238bb65f96415. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Cli/Configuration/FlexibleBooleanDictionaryConverter.cs:39

        }

        if (reader.TokenType != JsonTokenType.StartObject)
        {
            throw new JsonException($"Expected StartObject, got {reader.TokenType}");
        }

        var dictionary = new Dictionary<string, bool>();

        while (reader.Read())
        {
            if (reader.TokenType == JsonTokenType.EndObject)
            {
                return dictionary;
            }

            if (reader.TokenType != JsonTokenType.PropertyName)
            {
                throw new JsonException($"Expected PropertyName, got {reader.TokenType}");
            }

            var key = reader.GetString() ?? throw new JsonException("Property name cannot be null");

            reader.Read();

            bool value = reader.TokenType switch
            {
                JsonTokenType.True => true,
                JsonTokenType.False => false,
                JsonTokenType.String => ParseBooleanString(reader.GetString(), key),
                _ => throw new JsonException($"Cannot convert {reader.TokenType} to boolean for key '{key}'")
            };

            dictionary[key] = value;
        }

        throw new JsonException("Unexpected end of JSON");

View on GitHub (pinned to 25830f84bd)