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error in Microsoft.Extensions.Configuration

Was trying to follow a tutorial here for a console app https://www.connectionstrings.com/store-and-read-connection-string-in-appsettings-json/
So in my appsettings-json i have:
{
"ConnectionStrings": {
"myDb1": "Server=myServer;Database=myDb1;Trusted_Connection=True;",
}
}
My program:
using System;
using Microsoft.Extensions.Configuration;
namespace mynamespace
{
class Program
{
string myDb1ConnectionString = _configuration.GetConnectionString("myDb1");
static void Main(string[] args)
{
...
}
}
}
The error I get is : "The name '_configuration' does not exist in the current context".

The page you link to isn't a tutorial, it only shows how to read a connection string from any configuration provider, not just appsettings.json. It assumes you've already built a configuration object. .NET (Core) 5 and 6 use far simpler code though.
Check Configuration in .NET to understand how configuration really works. You can find more detailed information on the various config providers, how they're used and how to create your own in Configuration in ASP.NET Core
Using a generic host
In .NET 6, the current long term version, a minimal application would need:
using Microsoft.Extensions.Hosting;
using IHost host = Host.CreateDefaultBuilder(args).Build();
var configuration=host.Services.GetRequiredService<IConfiguration>();
var connectionString=configuration.GetConnectionString("blahblah");
...
As the docs explain, CreateDefaultBuilder will load configuration settings from any appsettings.json files, environment variables and finally command-line parameters.
This means you can override the settings stored in the JSON files by specifying the new values using environment variables or CLI parameters, eg :
dotnet run /ConnectionStrings:blahblah="......."
Without a generic host
You can create just the Configuration object by using a ConfigurationBuilder:
IConfiguration config = new ConfigurationBuilder()
.AddJsonFile("appsettings.json")
.AddEnvironmentVariables()
.AddCommandLine(args)
.Build();
var connectionString=config.GetConnectionString("blahblah");

'WebHost' is inaccessible due to its protection level

I am trying to follow Microsoft's Ocelot API Gateway tutorial (https://learn.microsoft.com/en-us/dotnet/architecture/microservices/multi-container-microservice-net-applications/implement-api-gateways-with-ocelot).
First I intialized a new empty ASP.NET Core web app:
dotnet new web
Then I installed the Ocelot dependencies (https://www.nuget.org/packages/Ocelot/):
dotnet add package Ocelot --version 17.0.0
Then I took the code from the tutorial:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Ocelot.DependencyInjection;
using Ocelot.Middleware;
using System.IO;
namespace MyApp
{
public class Program
{
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}
public static IWebHost BuildWebHost(string[] args)
{
var builder = WebHost.CreateDefaultBuilder(args);
builder.ConfigureServices(s => s.AddSingleton(builder))
.ConfigureAppConfiguration(
ic => ic.AddJsonFile(Path.Combine("configuration",
"configuration.json")))
.UseStartup<Startup>();
var host = builder.Build();
return host;
}
}
}
But then it complains that the WebHost class, called in BuildWebHost method, "is inaccessible due to its protection level". According to Microsoft (https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.webhost), WebHost "provides convenience methods for creating instances of IWebHost and IWebHostBuilder with pre-configured defaults.", and looks like so:
public static class WebHost
...
Why does it complain that WebHost is inaccessible, when the class is in fact public? What am I missing here?

From the documentation, WebHost is in the namespace Microsoft.AspNetCore. But in your code, It hasn't the using to this namespace.
In Visual Studio, you can try Go to definition on WebHost to discover where the type come.
As sujested by #leiflundgren, as your code has the using Microsoft.AspNetCore.Hosting, then the compiler thinks you want use Microsoft.AspNetCore.Hosting.WebHost.
https://github.com/dotnet/aspnetcore/blob/main/src/Hosting/Hosting/src/Internal/WebHost.cs
namespace Microsoft.AspNetCore.Hosting;
internal sealed partial class WebHost : IWebHost, IAsyncDisposable
{
....
}
But this class has the scope internal, then it isn't exposed and can be used by your code. Hence the following error :
WebHost is inaccessible due to its protection level.

Connect to Entity Framework Core in .NET 3.1 Azure Function Project Startup.cs

I am attempting to use entity framework against a database in Azure. I have a connection string stored in the local.settings.json, but all of my attempts to reference it from a Startup.cs have failed.
I am able to access my connection string using an IConfiguration DI into a function class and I can successfully access the database with SQL Command using the configuration like:
string connectionString = _configuration.GetConnectionString("cpni");
So I know that the connection string is working and that I can access it.
If I try to use DI with IConfiguration on the Startup class, the compiler does not give me any errors, but once it's running in debug I begin getting the following error:
System.Private.CoreLib: No parameterless constructor defined for type 'CPNI_Functions.Startup'.
Here is what I'm currently successfully using with a hardcoded connectionString (since using DI with IConfiguration isn't working):
builder.Services.AddDbContext<CpniContext>(
options => options.UseSqlServer(connectionString));
And that allows me to work with the database through EF. What I would like to use is some combination of that and something like this:
builder.Services.AddDbContext<CpniContext>(/*options need to go here?*/)
.Configure<IConfiguration>((configuration) =>
{
//Or are options supposed to go here somewhere, or be bound here with some sort of .Bind()?
configuration.GetConnectionString("cpni");
});
If that doesn't make sense or won't work, then please let me know what the recommended way of setting this DI up is. If possible, I want to avoid using configuration lookup through ConfigurationManager.
For reference, here is the full Startup.cs and this works:
using CPNI_Functions.Data;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
[assembly: FunctionsStartup(typeof(CPNI_Functions.Startup))]
namespace CPNI_Functions
{
class Startup : FunctionsStartup
{
public override void Configure(IFunctionsHostBuilder builder)
{
string connectionString = "myconnectionstringhere";
builder.Services.AddDbContext<CpniContext>(
options => options.UseSqlServer(connectionString));
}
/*
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContext<CpniContext>(configuration.GetConnectionString("cpni"));
}*/
}
}
I'm new to core and I'm new to Azure Functions, so please forgive my ignorance on this. Thank you in advance for your help.

For an Azure function app you could try adding NuGet package:
System.Configuration.ConfigurationManager
local.settings.json:
{
"ConnectionStrings": {
"cpni": "[ConnectionStringDetails]"
}
}
Then hopefully you should be able to access using ConfigurationManager.
services.AddDbContext<CpniContext>(
options => options.UseSqlServer(ConfigurationManager.ConnectionStrings["cpni"].ConnectionString));

Unable to create migrations after upgrading to ASP.NET Core 2.0

After upgrading to ASP.NET Core 2.0, I can't seem to create migrations anymore.
I'm getting
"An error occurred while calling method 'BuildWebHost' on class
'Program'. Continuing without the application service provider. Error:
One or more errors occurred. (Cannot open database "..." requested by
the login. The login failed. Login failed for user '...'"
and
"Unable to create an object of type 'MyContext'. Add an implementation
of 'IDesignTimeDbContextFactory' to the project, or see
https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns
supported at design time."
The command I previously ran was $ dotnet ef migrations add InitialCreate --startup-project "..\Web" (from the project/folder with the DBContext).
Connection string: "Server=(localdb)\\mssqllocaldb;Database=database;Trusted_Connection=True;MultipleActiveResultSets=true"
This is my Program.cs
public class Program
{
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}
public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>()
.Build();
}

You can add a class that implements IDesignTimeDbContextFactory inside of your Web project.
Here is the sample code:
public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<CodingBlastDbContext>
{
public CodingBlastDbContext CreateDbContext(string[] args)
{
IConfigurationRoot configuration = new ConfigurationBuilder()
.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json")
.Build();
var builder = new DbContextOptionsBuilder<CodingBlastDbContext>();
var connectionString = configuration.GetConnectionString("DefaultConnection");
builder.UseSqlServer(connectionString);
return new CodingBlastDbContext(builder.Options);
}
}
Then, navigate to your Database project and run the following from command line:
dotnet ef migrations add InitialMigration -s ../Web/
dotnet ef database update -s ../Web/
-s stands for startup project and ../Web/ is the location of my web/startup project.
resource

No need for IDesignTimeDbContextFactory.
Run
add-migration initial -verbose
that will reveal the details under
An error occurred while accessing the IWebHost on class 'Program'. Continuing without the application service provider.
warning, which is the root cause of the problem.
In my case, problem was, having ApplicationRole : IdentityRole<int> and invoking services.AddIdentity<ApplicationUser, IdentityRole>() which was causing below error
System.ArgumentException: GenericArguments[1], 'Microsoft.AspNetCore.Identity.IdentityRole',
on 'Microsoft.AspNetCore.Identity.EntityFrameworkCore.UserStore`9[TUser,TRole,TContext,
TKey,TUserClaim,TUserRole,TUserLogin,TUserToken,TRoleClaim]' violates the constraint of type 'TRole'.
---> System.TypeLoadException: GenericArguments[1], 'Microsoft.AspNetCore.Identity.IdentityRole',
on 'Microsoft.AspNetCore.Identity.UserStoreBase`8[TUser,TRole,TKey,TUserClaim,
TUserRole,TUserLogin,TUserToken,TRoleClaim]' violates the constraint of type parameter 'TRole'.

Solution 1: (Find the problem in 99% of cases)
Set Web Application project as Startup Project
Run the following commands with -verbose option.
Add-Migration Init -Verbose
-verbose option helps to actually uncover the real problem, It
contains detailed errors.
Solution 2:
Rename BuildWebHost() to CreateWebHostBuilder(), because Entity Framework Core tools expect to find a CreateHostBuilder method that configures the host without running the app.
.NET Core 2.2
public class Program
{
public static void Main(string[] args)
{
CreateWebHostBuilder(args).Build().Run();
}
public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>();
}
.NET Core 3.1
Rename BuildWebHost() to CreateHostBuilder()
public class Program
{
public static void Main(string[] args)
{
CreateHostBuilder(args).Build().Run();
}
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});
}
Solution 3:
Make sure you added Dbcontext to dependency injection:
AddDbContext<TContext> will make both your DbContext type, TContext, and the corresponding DbContextOptions<TContext> available for injection from the service container.
This requires adding a constructor argument to your DbContext type that accepts DbContextOptions<TContext>.
Example:
In Startup.cs
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContext<AppDbContext>(options => options.UseSqlServer(connectionString));
}
AppDbContext code:
public class AppDbContext: DbContext
{
public AppDbContext(DbContextOptions<AppDbContext> options)
:base(options)
{ }
}

public class Program
{
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}
public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>()
.Build();
}
}
Just rename BuildWebHost() to CreateWebHostBuilder(), because migrations use this method by default.

In my case, the cause of the problem was multiple startup projects. I have three projects in my solution: Mvc, Api, and Dal. DbContext and Migrations in the Dal project.
I had configured multiple startup projects. Both Mvc and Api projects were running when I clicked Start. But in this case I was getting this error.
"Unable to create an object of type 'MyContext'. Add an implementation
of 'IDesignTimeDbContextFactory' to the project, or see
https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns
supported at design time."
I could successfully add migration after setting Mvc as the only startup project and selecting Dal in the Package Manager Console.

In the AppContext.cs besides AppContext class add another class:
// required when local database deleted
public class ToDoContextFactory : IDesignTimeDbContextFactory<AppContext>
{
public AppContext CreateDbContext(string[] args)
{
var builder = new DbContextOptionsBuilder<AppContext>();
builder.UseSqlServer("Server=localhost;Database=DbName;Trusted_Connection=True;MultipleActiveResultSets=true");
return new AppContext(builder.Options);
}
}
This will solve your second problem:
"Unable to create an object of type 'MyContext'. Add an implementation of 'IDesignTimeDbContextFactory' to the project,
After that you will be able to add-migration Initial and execute it by running update-database command.
However if running these commands when there is no DataBase yet in your local SqlServer you will get the warning like your first error: "An error
occurred while calling method 'BuildWebHost' on class 'Program'... The
login failed. Login failed for user '...'"
But it is not error because migration will be created and it can be executed.
So just ignore this error for the first time, and latter since Db will exist it won't happen again.

You can try this solution from this discussion, which was inspired by this post.
public static IWebHost MigrateDatabase(this IWebHost webHost)
{
using (var scope = webHost.Services.CreateScope())
{
var services = scope.ServiceProvider;
try
{
var db = services.GetRequiredService<MyContext>();
db.Database.Migrate();
}
catch (Exception ex)
{
var logger = services.GetRequiredService<ILogger<Program>>();
logger.LogError(ex, "An error occurred while migrating the database.");
}
}
return webHost;
}
public static void Main(string[] args)
{
BuildWebHost(args)
.MigrateDatabase()
.Run();
}

Something that really helped me was this article: https://elanderson.net/2017/09/unable-to-create-an-object-of-type-applicationdbcontext-add-an-implementation-of-idesigntimedbcontextfactory/
The basic idea is that in the change over from .net core 1 to 2 all db initialization should be moved out of the StartUp.cs and into the Program.cs. Otherwise the EF tasks try and run your DB inits when doing tasks.
"There is a nice section in the official migration docs (https://learn.microsoft.com/en-us/ef/core/miscellaneous/1x-2x-upgrade) titled “Move database initialization code” which I seemed to have missed. So before you head down any rabbit holes like I did make sure this isn’t what is causing your need to add an implementation of IdesignTimeDbContextFactory."

Please verify that you have the reference
<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="2.0.0" />

From
https://learn.microsoft.com/en-us/ef/core/miscellaneous/cli/dbcontext-creation
When you create a new ASP.NET Core 2.0 application, this hook is
included by default. In previous versions of EF Core and ASP.NET Core,
the tools try to invoke Startup.ConfigureServices directly in order to
obtain the application's service provider, but this pattern no longer
works correctly in ASP.NET Core 2.0 applications. If you are upgrading
an ASP.NET Core 1.x application to 2.0, you can modify your Program
class to follow the new pattern.
Add Factory in .Net Core 2.x
public class BloggingContextFactory : IDesignTimeDbContextFactory<BloggingContext>
{
public BloggingContext CreateDbContext(string[] args)
{
var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
optionsBuilder.UseSqlite("Data Source=blog.db");
return new BloggingContext(optionsBuilder.Options);
}
}

I had this problem and this solved By Set -> Web Application(Included Program.cs) Project to -> "Set as Startup Project"
Then run -> add-migration initial -verbose
in Package Manager Console
Set as Startup Project

If you want to avoid those IDesignTimeDbContextFactory thing: Just make sure that you don't use any Seed method in your startup. I was using a static seed method in my startup and it was causing this error for me.

I was facing the error
"Unable to create an object of type 'MyContext'. Add an implementation of 'IDesignTimeDbContextFactory' to the project, or see https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns supported at design time."
This is how my problem was solved. Run the below command while you are in your solution directory
dotnet ef migrations add InitialMigration --project "Blog.Infrastructure" --startup-project "Blog.Appication"
Here Application is my startup project containing the Startup.cs class & Infrastructure is my project containing the DbContext class.
then run update using the same structure.
dotnet ef database update --project "Blog.Infrastructure" --startup-project "Blog.Application"

Previously, you configured the seed data in the Configure method in Startup.cs. It is now recommended that you use the Configure method only to set up the request pipeline. Application startup code belongs in the Main method.
The refactored Main method. Add the following references to the Program.cs:
using Microsoft.Extensions.DependencyInjection;
using MyProject.MyDbContextFolder;
public static void Main(string[] args)
{
var host = BuildWebHost(args);
using (var scope = host.Services.CreateScope())
{
var services = scope.ServiceProvider;
try
{
var context = services.GetRequiredService<MyDbConext>();
DbInitializer.Initialize(context);
}
catch (Exception ex)
{
var logger = services.GetRequiredService<ILogger<Program>>();
logger.LogError(ex, "An error occurred while seeding the database.");
}
}
host.Run();
}

There's a problem with ef seeding db from Startup.Configure in 2.0 ... you can still do it with this work around. Tested and worked fine
https://garywoodfine.com/how-to-seed-your-ef-core-database/

In my case I got the problem because I had a method named SeedData.EnsurePopulated() being called on my Startup.cs file.
public class Startup
{
public Startup(IConfiguration configuration) => Configuration = configuration;
public IConfiguration Configuration { get; }
public void ConfigureServices(IServiceCollection services)
{
//
}
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
app.UseDeveloperExceptionPage();
app.UseStatusCodePages();
app.UseStaticFiles();
app.UseSession();
app.UseMvc(routes =>
{
//
});
SeedData.EnsurePopulated(app);
}
}
The work of SeedData class is to add initial data to the database table. It's code is:
public static void EnsurePopulated(IApplicationBuilder app)
{
ApplicationDbContext context = app.ApplicationServices.GetRequiredService<ApplicationDbContext>();
context.Database.Migrate();
if (!context.Products.Any())
{
context.Products.AddRange(
new Product
{
Name = "Kayak",
Description = "A boat for one person",
Category = "Watersports",
Price = 275
},
....
);
context.SaveChanges();
}
}
SOLUTION
Before doing migration simply comment out the calling of SeedData class in the Startup.cs file.
// SeedData.EnsurePopulated(app);
That solved my problem and hope your problem is also solved in the same way.

I ran into same problem. I have two projects in the solution. which
API
Services and repo, which hold context models
Initially, API project was set as Startup project.
I changed the Startup project to the one which holds context classes.
if you are using Visual Studio you can set a project as Startup project by:
open solution explorer >> right-click on context project >> select Set as Startup project

First of all make sure you have configured your database in Startup.cs
In my case, i was getting this error since i didn't specify the below in Startup.cs
services.AddDbContext<ApplicationDbContext>(options =>
options.UseSqlServer(
Configuration.GetConnectionString("DefaultConnection"), x => x.MigrationsAssembly("<Your Project Assembly name where DBContext class resides>")));

Using ASP.NET Core 3.1 and EntityFrameWorkCore 3.1.0. Overriding the OnConfiguring of the context class with a parameterless constructor only
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
if (!optionsBuilder.IsConfigured)
{
IConfigurationRoot configuration = new ConfigurationBuilder()
.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json")
.Build();
var connectionString = configuration.GetConnectionString("LibraryConnection");
optionsBuilder.UseSqlServer(connectionString);
}
}

I got the same issue since I was referring old- Microsoft.EntityFrameworkCore.Tools.DotNet
<DotNetCliToolReference Include="Microsoft.EntityFrameworkCore.Tools.DotNet" Version="1.0.0" />
After upgrading to the newer version it got resolved

In main project's appsettings.json file, I had set 'Copy to Output directory' to "Copy always" and it worked.

Sample DB context class for .net core console applications
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;
using Microsoft.Extensions.Configuration;
using System.IO;
namespace EmailServerConsole.Data
{
public class EmailDBContext : DbContext
{
public EmailDBContext(DbContextOptions<EmailDBContext> options) : base(options) { }
public DbSet<EmailQueue> EmailsQueue { get; set; }
}
public class ApplicationContextDbFactory : IDesignTimeDbContextFactory<EmailDBContext>
{
EmailDBContext IDesignTimeDbContextFactory<EmailDBContext>.CreateDbContext(string[] args)
{
IConfigurationRoot configuration = new ConfigurationBuilder()
.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json")
.Build();
var builder = new DbContextOptionsBuilder<EmailDBContext>();
var connectionString = configuration.GetConnectionString("connection_string");
builder.UseSqlServer(connectionString);
return new EmailDBContext(builder.Options);
}
}
}

You also can use in the startup class constructor to add json file (where the connection string lies) to the configuration. Example:
IConfigurationRoot _config;
public Startup(IHostingEnvironment env)
{
var builder = new ConfigurationBuilder()
.SetBasePath(env.ContentRootPath)
.AddJsonFile("appsettings.json");
_config = builder.Build();
}

For me it was because I changed the Output Type of my startup project from Console Application to Class Library.
Reverting to Console Application did the trick.

I had this issue in a solution that has:
a .NET Core 2.2 MVC project
a .NET Core 3.0 Blazor project
The DB Context in a .NET Standard 2.0 class library project
I get the "unable to create an object..." message when the Blazor project is set as the start up project, but not if the MVC project is set as the startup project.
That puzzles me, because in the Package Manager Console (which is where I'm creating the migration) I have the Default project set to a the C# class library that actually contains the DB Context, and I'm also specifying the DB context in my call to add-migration add-migration MigrationName -context ContextName, so it seems strange that Visual Studio cares what startup project is currently set.
I'm guessing the reason is that when the Blazor project is the startup project the PMC is determining the version of .NET to be Core 3.0 from the startup project and then trying to use that to run the migrations on the .NET Standard 2.0 class library and hitting a conflict of some sort.
Whatever the cause, changing the startup project to the MVC project that targets Core 2.2, rather than the Blazor project, fixed the issue

For me the problem was that I was running the migration commands inside the wrong project. Running the commands inside the project that contained the Startup.cs rather than the project that contained the DbContext allowed me to move past this particular problem.

In my case setting the StartUp project in init helps. You can do this by executing
dotnet ef migrations add init -s ../StartUpProjectName

Manzur Alahi is right! I'm trying to learn Rider by JetBrains and I had the same error when I was trying to use dotnet-ef migrations add ... in Cmd, PowerShell, etc. but when I used Visual Studio IDE I didn't have problem.
I fixed the error with:
dotnet ef migrations add InitialMigration --project "Domain.Entities" --startup-project "WebApi"
and this to update the database
dotnet ef database update --project "Domain.Entities" --startup-project "WebApi"
just like Manzur Alahi said.

If context class is in another class library project and this error is occurred, change command line default project to the context project and set solution startup project to the main API / ASP.net core project (that your DI container is there), then re-run command
It seems ef core tools package has this bug a reported in
https://github.com/dotnet/efcore/issues/23957 and https://github.com/dotnet/efcore/issues/23853

I had same problem. Just changed the ap.jason to application.jason and it fixed the issue

Can't make a function call in in spite of the using-statement and extension needed

So I wanted to be able to choose my environment when running dotnet (an .net core mvc-project) from the terminal. I found this post and thought the second highest answer was a neat way of solving it, in short:
Replacing the Program class body with the following:
private static readonly Dictionary<string, string> defaults =
new Dictionary<string, string> {
{ WebHostDefaults.EnvironmentKey, "development" }
};
public static void Main(string[] args)
{
var configuration =
new ConfigurationBuilder()
.AddInMemoryCollection(defaults)
.AddEnvironmentVariables("ASPNETCORE_")
.AddCommandLine(args)
.Build();
var host = new WebHostBuilder()
.UseKestrel()
.UseContentRoot(Directory.GetCurrentDirectory())
.UseIISIntegration()
.UseStartup<Startup>()
.Build();
host.Run();
}
Then running:
dotnet run environment=development
dotnet run environment=staging
So I pasted it, and it said I need to add a using statment
using Microsoft.Extensions.Configuration;
Still got this error message:
'IConfigurationBuilder' does not contain a definition for
'AddCommandLine' and no extension method 'AddCommandLine'
accepting a first argument of type 'IConfigurationBuilder'
could be found (are you missing a using directive or an
assembly reference?)
I'm a bit at loss for what could be the problem. I mean, here's the definition of AddCommandLine(IConfigurationBuilder, String[])
with namespace Microsoft.Extensions.Configuration?

Although the namespace is Microsoft.Extensions.Configuration, the extension is in the Microsoft.Extensions.Configuration.CommandLine assembly, which is in the Microsoft.Extensions.Configuration.CommandLine package. You need to add a dependency on that package.