Read environment variables in ASP.NET Core

Your problem is spaces around =.

This will work (attention to space before closing quote):

Console.WriteLine(Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT "));

The space after ASPNETCORE_ENVIRONMENT in this code is not a typo! The problem in the question was having extra space (in SET...), so you must use the same space in GetEnvironmentVariable().

As noted by Isantipov in a comment, an even better solution is to remove the spaces entirely from the SET command:

set ASPNETCORE_ENVIRONMENT=Production

If you create the Environment variables at runtime during development then you will get null every time. You have to restart the visual studio because VS read EV at startup only.

This should really be a comment to this answer by @Dmitry (but it is too long, hence I post it as a separate answer):

You wouldn't want to use 'ASPNETCORE_ENVIRONMENT ' (with a trailing space) - there are features in ASP.NET Core which depend on the value of 'ASPNETCORE_ENVIRONMENT'(no trailing space) - e.g. resolving of appsettings.Development.json vs appsettings.Production.json. (e.g. see Working with multiple environments documentation article

And also I guess if you'd like to stay purely inside ASP.NET Core paradigm, you'd want to use IHostingEnvironment.Environment(see documentation) property instead of reading from Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") directly (although the former is of course set from the latter). E.g. in Startup.cs

public class Startup
{
    //<...>

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        Console.WriteLine("HostingEnvironmentName: '{0}'", env.EnvironmentName);
        //<...>
    }

    //<...>
}