I was just doing a reply, but you beat me to it! For anyone that looks at this in the future this is how it should be done in .NET:
That's how you would type it in VBA. Sorry, I should have put it in .NET for you.
Excel.Application.Sheets.Add(Type.Missing,Type.Missing, Type.Missing, "TemplateWorkbookPath")
I'm not sure what would be the equivalent of Type.Missing in C#, that is in VB.