How to import a namespace in Razor View Page?
Depending on your need you can use one of following method:
if required in all subsequent views then add "using your.domainName;" in _ViewStart.cshtml. You can find more about this in: Where and how is the _ViewStart.cshtml layout file linked?
Or add Assembly reference in View web.config as described by others explained in: How do you implement a @using across all Views in Asp.Net MVC 3?