Class not updated in g.i.cs file

I have this class

public class Questionnaire
{
    public List<Question> Questions { get; set; }

    public Question this[int i]
    {
        get { return Questions[i]; }
    }
}

And I'm trying to use it on my page with

Questionnaire questionnaire;

protected override async Task OnInitAsync()
{
    questionnaire.Questions = new List<Question>();
}

But I just keep getting

'Questionnaire' does not contain a definition for 'Questions' and no extension method 'Questions' accepting a first argument of type 'Questionnaire' could be found (are you missing a using directive or an assembly reference?)

I noticed that I can seem to call

questionnaire.QuestionNumber = 1;

Even though I removed the QuestionNumber property ages ago.

Following the reference I can see that in Questionnaire.g.i.cs there is a QuestionNumber property but no Questions property.

I tried restarting VS, cleaning project, rebuilding the project but this achieved nothing.