HttpContext.Current.User.Identity.Name returns null or empty string

This error come due to authentication mode problem.

To correct this  error follow the following step:

1. Select your project.
2. press F4.
3. the screen will open like the below image.
4. disable the "Anonymous Authentication" and enable the "Window Authentication".

paste in the web configuration

<system.web>
    <authentication mode="Windows">
</system.web>