Optional parameters for method calls not supported by expression evaluation

STR

Define a rule as following:

{
      "RuleName": "TestPdfAttachmentContent1",
      "Expression": "pdfAttachments[0].PdfContentContainsSubstring(\"Mahnung\")"
}

(That input/RuleParameter is of course defined, it's not what matters here.)

If I catch all CLR exceptions while debugging, I get the following exception:

RulesEngine.Exceptions.RuleException
  HResult=0x80131500
  Nachricht = Error while compiling rule `TestPdfAttachmentContent1`: No applicable method 'PdfContentContainsSubstring' exists in type 'PdfAttachmentValidator'
  Quelle = RulesEngine
  Stapelüberwachung:
   bei RulesEngine.HelperFunctions.Helpers.HandleRuleException(Exception ex, Rule rule, ReSettings reSettings)

  Diese Ausnahme wurde ursprünglich von dieser Aufrufliste ausgelöst:
    RulesEngine.HelperFunctions.Helpers.HandleRuleException(System.Exception, RulesEngine.Models.Rule, RulesEngine.Models.ReSettings)
    RulesEngine.ExpressionBuilders.LambdaExpressionBuilder.BuildDelegateForRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.BuildRuleFunc(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.GetDelegateForRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.CompileRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleExpressionType, RulesEngine.Models.RuleParameter[], System.Lazy<RulesEngine.Models.RuleExpressionParameter[]>)

Innere Ausnahme 1:
ParseException: No applicable method 'PdfContentContainsSubstring' exists in type 'PdfAttachmentValidator'

Not, however, and this is a general behavior I don't quite like the exception is silently ignored even though I have explicitly disabled EnableExceptionAsErrorMessage (see below).

Note this is not true, because the PdfAttachmentValidator has such a method, but with an optional second argument:

public class PdfAttachmentValidator
{
    public bool PdfContentContainsSubstring(string substring, bool caseInsensitive = true)
    {
        return true;
    }
}

And yes, I've obviously injected the type:

      var engineSettings = new ReSettings()
        {
            CustomActions = // ...
            CustomTypes =
            [
                typeof(PdfAttachmentValidator)
            ],
            EnableExceptionAsErrorMessage = false
        };
     
        new RulesEngine.RulesEngine(mailWorkflows, engineSettings);

What happens

Apparently, it cannot find PdfContentContainsSubstring("Mahnung") as it cannot find the method signature PdfContentContainsSubstring(string substring, bool caseInsensitive = true).

What should happen

Just call PdfContentContainsSubstring with PdfContentContainsSubstring("Mahnung"), ignoring the optional parameter.

Workaround

Changing it like this and passing the optional parameter explicitly, makes it magically work:

{
      "RuleName": "TestPdfAttachmentContent1",
      "Expression": "pdfAttachments[0].PdfContentContainsSubstring(\"Mahnung\", true)"
}

But this is superfluous, obviously. And I would like to avoid this for obvious reasons.

More information

Weirdly enough, even changing it to default does not work (for whatever reason):

{
      "RuleName": "TestPdfAttachmentContent1",
      "Expression": "pdfAttachments[0].PdfContentContainsSubstring(\"Mahnung\", default)"
}

The (handled) exception then is Unknown identifier 'default':

RulesEngine.Exceptions.RuleException
  HResult=0x80131500
  Nachricht = Error while compiling rule `TestPdfAttachmentContent1`: Unknown identifier 'default'
  Quelle = RulesEngine
  Stapelüberwachung:
   bei RulesEngine.HelperFunctions.Helpers.HandleRuleException(Exception ex, Rule rule, ReSettings reSettings)
   bei RulesEngine.HelperFunctions.Helpers.ToRuleExceptionResult(ReSettings reSettings, Rule rule, Exception ex)
   bei RulesEngine.RuleCompiler.CompileRule(Rule rule, RuleExpressionType ruleExpressionType, RuleParameter[] ruleParams, Lazy`1 globalParams)
   bei RulesEngine.RulesEngine.CompileRule(Rule rule, RuleExpressionType ruleExpressionType, RuleParameter[] ruleParams, Lazy`1 scopedParams)
   bei RulesEngine.RulesEngine.CompileRule(String workflowName, String ruleName, RuleParameter[] ruleParameters)
   bei RulesEngine.RulesEngine.<ExecuteActionWorkflowAsync>d__13.MoveNext()

  Diese Ausnahme wurde ursprünglich von dieser Aufrufliste ausgelöst:
    RulesEngine.HelperFunctions.Helpers.HandleRuleException(System.Exception, RulesEngine.Models.Rule, RulesEngine.Models.ReSettings)
    RulesEngine.ExpressionBuilders.LambdaExpressionBuilder.BuildDelegateForRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.BuildRuleFunc(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.GetDelegateForRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleParameter[])
    RulesEngine.RuleCompiler.CompileRule(RulesEngine.Models.Rule, RulesEngine.Models.RuleExpressionType, RulesEngine.Models.RuleParameter[], System.Lazy<RulesEngine.Models.RuleExpressionParameter[]>)

Innere Ausnahme 1:
ParseException: Unknown identifier 'default'

System

    <TargetFramework>net8.0</TargetFramework>
<!-- ... -->
    <PackageReference Include="RulesEngine" Version="5.0.3" /> 

aka .NET 8

Windows 10

Start by reproducing the expression with PdfAttachmentValidator and an omitted optional argument, then inspect LambdaExpressionBuilder.BuildDelegateForRule and the RuleCompiler entry points named in the stack trace. Done means the expression compiles and invokes PdfContentContainsSubstring("Mahnung") without explicitly passing the optional parameter, with regression coverage for this case.