Schema printer writes value-type argument defaults unquoted (Guid, DateTime, …) and culture-dependent #562

What happens

SchemaGenerator.GetArgDefaultValue prints any value type with ToString(), unquoted:

else if (valueType.IsValueType)
{
    return defaultArgValue.Value.ToString();
}

An arguments object with a Guid property therefore produces invalid SDL:

public sealed class RunMessagesArgs
{
    public Guid RunId { get; set; }
}

query.AddField("runMessages", new RunMessagesArgs(), (db, args) => /* … */);

runMessages(runId: ID! = 00000000-0000-0000-0000-000000000000): RunMessagePage

GraphQL Code Generator then refuses the schema: Syntax Error: Invalid number, unexpected digit after 0: "0".

Other cases on the same path

• String-like value types are printed bare as well: DateTime, DateTimeOffset, DateOnly, TimeOnly, TimeSpan and char. Each should be a quoted string literal.
• Floating point and decimal values use the current culture. Under de-DE, a default of 3.14 prints as 3,14. TestGetArgDefaultValue_Decimal passes only because of the culture the tests run under.
• String defaults are not escaped. $"\"{value}\"" breaks on a default containing ", \ or a newline.

Suggested fix

Format by type:

• quote and escape strings and the string-like value types above;
• format numbers with CultureInfo.InvariantCulture;
• leave bool and enums as they are today.

Add cases for Guid, DateTime, a string containing a quote, and a decimal under a non-invariant culture to ToGraphQLSchemaStringTests.

Related (possibly a separate issue)

ExpressionUtil.ObjectToDictionaryArgs treats any non-null property value as a declared default (new DefaultArgValue(value != null, value)). A value type's implicit default(T) therefore becomes a schema default: int Turn becomes turn: Int! = 0. Once the quoting is fixed, a Guid becomes ID! = "00000000-…". That is valid SDL, but it turns a required argument into an optional one for clients.

Changing this would break anyone who relies on an implicit 0 making an argument optional. So it probably needs to be opt-in or wait for a major version. The current workaround is a Guid? property marked [GraphQLNotNull].