Razor View Without Layout

I think this :

@{
    Layout = "";
 }

is not the same as this :

@{
    Layout = null;
 }

I use the second and it's working, no _Viewstart included.

You (and KMulligan) are misunderstanding _ViewStart pages.

_ViewStart will always execute, before your page starts.
It is intended to be used to initialize properties (such as Layout); it generally should not contain markup. (Since there is no way to override it).

The correct pattern is to make a separate layout page which calls RenderBody, and set the Layout property to point to this page in _ViewStart.

You can then change Layout in your content pages, and the changes will take effect.