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

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);
        //<...>
    }

    //<...>
}

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.