Question

I am trying to run a simple application that uses drool.net. Whenever I execute, the application crashed at builder.AddPackageFromDrl("DroolsApp.Rules.drl", stream); The Error stated that NullReferenceException was unhandled and Object reference not set an instance of an object. I tried to change drl file but the error was still present. Am I missing anything? Also is there any site that provides a detailed tutorial for using drools.net and do I need any specific program to create and edit drl files as I am using a simple notepad.

Thanks in Advance

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using org.drools.dotnet.compiler;
using org.drools.dotnet.rule;
using org.drools.dotnet;
using System.IO;
using System.Reflection;

namespace DroolsApp
{
    class Program
    {   
        static void Main(string[] args)
        {            
            PackageBuilder builder = new PackageBuilder();
            Stream stream = Assembly.GetExecutingAssembly()
            .GetManifestResourceStream("DroolsApp.Rules.drl");
            builder.AddPackageFromDrl("DroolsApp.Rules.drl", stream);
            Package pkg = builder.GetPackage();
            RuleBase ruleBase = RuleBaseFactory.NewRuleBase();
            ruleBase.AddPackage(pkg);            
            executeRules(ruleBase, 5);
            executeRules(ruleBase, 6);
            executeRules(ruleBase, 7);
        }

        public static void executeRules(RuleBase ruleBase, int amount)
        {
            WorkingMemory workingMemory = ruleBase.NewWorkingMemory();
            Quest quest = new Quest();
            quest.Value = amount;
            workingMemory.assertObject(quest);
            workingMemory.fireAllRules();
        }
    }
}   

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DroolsApp
{
    class Quest
    {
        public int Value { set; get; }
    }
}

DRL File:    

package DroolsApp

rule "NumberFive"
    when
        quest : Quest( Value == 5 )
    then
            Console.WriteLine("FIVE");
end 

rule "NumberSix"
    when
        quest : Quest( Value == 6 )
    then
            Console.WriteLine("SIX");
end

Stack Trace:

System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=drools.dotnet
  StackTrace:
       at org.drools.dotnet.semantics.DotnetClassTypeResolver.resolveType(String className)
       at org.drools.semantics.java.RuleBuilder.build(ColumnDescr columnDescr)
       at org.drools.semantics.java.RuleBuilder.build(RuleDescr ruleDescr)
       at org.drools.semantics.java.RuleBuilder.build(Package pkg, RuleDescr ruleDescr)
       at org.drools.compiler.PackageBuilder.addRule(RuleDescr ruleDescr)
       at org.drools.compiler.PackageBuilder.addPackage(PackageDescr packageDescr)
       at org.drools.compiler.PackageBuilder.addPackageFromDrl(String fileName, Reader reader)
       at org.drools.dotnet.compiler.PackageBuilder.AddPackageFromDrl(String fileName, Stream drlStream)
       at MinimalDroolsForm.Program.Main(String[] args) in C:\Users\Carl\Desktop\DroolsApp\Program.cs:line 19
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Answer 1

It’s for sure an error in drl file. Try to remove all rules and imports, compile them, and then add single line and repeat (it must validate though). Drools.net reacts for compilation errors with NullReferenceException or an error with loading CompiledRules0.dll (because it wasn’t generated). It is also a good thing to add a Pre-build Build Event deleting the previously generated CompiledRules0.dll (when Drools.net fails to generate the file it uses the previous one). Here’s the code:

cd $(TargetDir)
del CompiledRules0.*