How to get all keywords inside the particular category using C# TBB

Question

I have Tcm id of category in my app.config. In my C# TBB i am writing below code to get all keywords associated with this category

public class GetKeywords : ITemplate
  {
    public void Transform(Engine engine, Package package)
      {
        string categoryID =ConfigurationManager.AppSettings["catgoryTcmUri"];

        Category cat = (Category)engine.GetObject(categoryID);

        XmlDocument keywordsXml = new XmlDocument();
        keywordsXml.LoadXml(cat.GetListKeywords().OuterXml);
        Item output = package.CreateXmlDocumentItem(ContentType.Xml, keywordsXml);
        package.PushItem("CategoryKeywords", output);
    }
  }

I am getting follwing error in Template Builder:

 Object reference not set to an instance of an object.
 at Tieto.Gadgets.Templating.GetKeywords.Transform(Engine engine, Package package)
 at Tridion.ContentManager.Templating.Assembly.AssemblyMediator.Transform(Engine engine, Template template, Package package)
 at Tridion.ContentManager.Templating.Assembly.CSharpSourceCodeMediator.RunTemplate(Engine engine, Package package, String templateUri, String className)
 at Tridion.Templating.CSharpTemplate.CSharpSourceTemplate.Transform(Engine __engine, Package __package)
 at Tridion.ContentManager.Templating.Assembly.CSharpSourceCodeMediator.Transform(Engine engine, Template template, Package package)
 at Tridion.ContentManager.Templating.Engine.ExecuteTemplate(Template template, Package package)
 at Tridion.ContentManager.Templating.Engine.InvokeTemplate(Package package, TemplateInvocation templateInvocation, Template template)
 at Tridion.ContentManager.Templating.Compound.CompoundTemplateMediator.Transform(Engine engine, Template templateToTransform, Package package)
 at Tridion.ContentManager.Templating.Engine.ExecuteTemplate(Template template, Package package)
 at Tridion.ContentManager.Templating.Engine.InvokeTemplate(Package package, TemplateInvocation templateInvocation, Template template)
 at Tridion.ContentManager.Templating.Engine.TransformItem(Template template, IdentifiableObject itemToRender)
 at Tridion.ContentManager.Templating.Debugging.DebuggingEngine.Run()
 at Tridion.ContentManager.Templating.Debugging.DebugSession.Run()

Answer 1

Here are few things you should cross check as the code looks fine.

• As Pankaj already suspected , you might be using some config which your C# TBB won't be able to read/access , use the categoryID directly in your code first.
• You can debug your TBB using Logging or attaching to your Template Builder Process TcmTemplateDebugHost.exe either on the local machine or via remote debugging.
• See some related stuff here :https://stackoverflow.com/questions/12210711/tridion-template-builder-and-visual-studio-debugging

Answer 2

Use Template Builder to debug you code by adding TemplatingLogger lines between your commands.

public class GetKeywords : ITemplate
  {

    TemplatingLogger log = TemplatingLogger.GetLogger(this.GetType());

    public void Transform(Engine engine, Package package)
      {
        string categoryID = ConfigurationManager.AppSettings["catgoryTcmUri"];

        log.Debug("categoryID: " + categoryID)

        Category cat = (Category)engine.GetObject(categoryID);

        XmlDocument keywordsXml = new XmlDocument();
        keywordsXml.LoadXml(cat.GetListKeywords().OuterXml);
        Item output = package.CreateXmlDocumentItem(ContentType.Xml, keywordsXml);
        package.PushItem("CategoryKeywords", output);
    }
  }

When you run your Component Template or Page Template that has the C# TBB your are mentioning you can exactly localize the line that is raising an error. When you know the line this is 90% of your solution.

PS. I can't be sure but probably categoryID is in you case empty