Why have both _ViewStart and _ViewImports? Why not one file?

Question

In ASP.NET Core MVC we can put a file with the exact name of _ViewStart.cshtml inside a folder to contain the common C# code to be run before every razor view/page in that folder. Something like this:

@{
    const string SomeConstant = "some value";
}

Similarly a file with the exact name of _ViewImports.cshtml inside a folder can hold all the common razor directives to be shared among the razor views/pages in that folder. Like this:

@layout _Layout
@using MyApp.Models
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

But here's a question that I couldn't google, no matter how I rephrased it:

Can somebody please explain to me why we have both a _ViewStart.cshtml and a _ViewImports.cshtml to define common code & directives? Why aren't these functionalities (which don't seem to be conflicting with each-other) defined in a single file?

Answer 1

The _ViewStart file

It is used to set up shared-memory (public static variables) across all view files.

For example, the common practice for ViewStart is to set up a default value that you can override for the Layout and the ViewData / ViewBag dictionary.

The _ViewImports file

In this file you can summarize (abstract) all using statements that you commonly use in all your views.

Why to use _ViewImports file for common "using directives" instead of ViewStart?

Because using directives has the scope of the body of the current view file. So, putting @using statements inside ViewStart file won't make them available for any other view file except the body of the viewStart file itself. Therefore, comes the special ViewImports file which is designed to serve this scope extension purpose of the @using statements and other useful things, such as the tag helper, which without this special file, would be repeated inside each view file which violates the DRY (Don't Repeat Yourself) Principle.

Answer 2

One thing that has been overlooked in the other answers is that according to the official documentation:

• Code in the _ViewStart.cshtml file will only be run for non-layout pages.
• Code in the _ViewImports.cshtml file will be run for both layout and non-layout pages.

I've tested this by moving the default Application Insights JavaScript snippet (the code below) from the imports file to the start file and it causing a build error on my layout page as it can no longer find the defined variable JavaScriptSnippet.

The code I moved:

@inject Microsoft.ApplicationInsights.AspNetCore.JavaScriptSnippet JavaScriptSnippet

Given this, the difference between the files is probably 'code I want to run everywhere' vs 'code I want to only run for full views', similar to the difference between .bashrc and .bash_profile.