asp.net core 2.0 - Value cannot be null. Parameter name: connectionString

Question

I had the following error in package manager console when Add-Migration

Value cannot be null. Parameter name: connectionString

Answer 1

I had a similar problem when I specified the ".UseContentRoot" as the current process path.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:3001")
            .UseStartup()
            .UseContentRoot(Path.GetDirectoryName(Process.GetCurrentProcess().MainModule.FileName));

thus when running Add-Migration the process path is different from the project bin path therefore the process can't find the appsettings.json file. when I removed the ".UseContentRoot" line the migration was successful