Umbraco - how to set property value using razor script

I am trying to create a very simple survey form, and I thought about using it instead of a razor instead of external control.

I created a form that lists one question and a list of answers, and when we click the submit button, we go to a page that calls a razor script that processes the results.

What I want to do is to look through all the answers and increase the counter by one. There is a numerical property called “Counter” for each “Response” object.

However, this continues to fail. If I do this:

var objAnswer = @Model.NodeById(Int32.Parse(submittedAnswer));
objAnswer.getProperty("Counter").Value++;

or similar methods, they all fail. It is strange that objAnswer.getProperty ("Counter") contains a number, but when I try to set it, I get this error:

   umbraco.MacroEngines.DynamicNull' does not contain a definition for 'Value'

I get I also tried using

Document post = new Document(objAnswer.Id);
post.Publish(user);

but it also fails.

Is there an easy way to achieve this?

The answer is:

      Document doc = new Document(objAnswer.Id);
      doc.getProperty("counter").Value = 34;
      umbraco.BusinessLogic.User author = umbraco.BusinessLogic.User.GetUser(0); 
      doc.Publish(author);
      umbraco.library.UpdateDocumentCache(doc.Id);

This does not mean that the property should be lowercase.