Get ConnectionString from appsettings.json instead of being hardcoded in .NET Core 2.0 App

I have the following class in NET Core2.0 App.

// required when local database does not exist or was 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 is required in Core 2.0 with migration when Database does not exist and has to be created when you run update-database.
Unable to create migrations after upgrading to ASP.NET Core 2.0

I would like not having ConnectionString in 2 places(here and in appsettings.json) but only in .json so I have tried to replace

"Server=localhost;Database=DbName;Trusted_Connection=True;MultipleActiveResultSets=true"

with

ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString

but it's not working, I'm getting null value.

UPDATE 1:
Just to note that adding explicitly .json is not necessery in Core 2 so the problem is not with the file.
https://andrewlock.net/exploring-program-and-startup-in-asp-net-core-2-preview1-2/

UPDATE 2:
Also I am already using Configuration for sending ConnectionString from .json to Context:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<AppContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    }
}

But I can not use this for ToDoContextFactory because it does not have Configuration, and ToDoContextFactory is used by migrations so the App is not running at all.

SOLUTION: Based on answer from @JRB I made it work like this:

public AppContext CreateDbContext(string[] args)
{
    string projectPath = AppDomain.CurrentDomain.BaseDirectory.Split(new String[] { @"bin\" }, StringSplitOptions.None)[0];
    IConfigurationRoot configuration = new ConfigurationBuilder()
        .SetBasePath(projectPath)
        .AddJsonFile("appsettings.json")
        .Build();
    string connectionString = configuration.GetConnectionString("DefaultConnection");

    var builder = new DbContextOptionsBuilder<AppContext>();
    builder.UseSqlServer(connectionString);

    return new AppContext(builder.Options);
}

STEP 1: Include the following in OnConfiguring()

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        IConfigurationRoot configuration = new ConfigurationBuilder()
            .SetBasePath(AppDomain.CurrentDomain.BaseDirectory)
            .AddJsonFile("appsettings.json")
            .Build();
        optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"));
    }

STEP 2: Create appsettings.json:

  {
    "ConnectionStrings": {       
      "DefaultConnection": "Server=YOURSERVERNAME; Database=YOURDATABASENAME; Trusted_Connection=True; MultipleActiveResultSets=true"        
    } 
  }

STEP 3: Hard copy appsettings.json to the correct directory

  Hard copy appsettings.json.config to the directory specified in the AppDomain.CurrentDomain.BaseDirectory directory. 
  Use your debugger to find out which directory that is.        

Assumption: you have already included package Microsoft.Extensions.Configuration.Json (get it from Nuget) in your project.

In ASPNET Core you do it in Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<BloggingContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("BloggingDatabase")));
}

where your connection is defined in appsettings.json

{
  "ConnectionStrings": {
    "BloggingDatabase": "..."
  },
}

Example from MS docs

There are a couple things missing, both from the solutions above and also from the Microsoft documentation. If you follow the link to the GitHub repository linked from the documentation above, you'll find the real solution.

I think the confusion lies with the fact that the default templates that many people are using do not contain the default constructor for Startup, so people don't necessarily know where the injected Configuration is coming from.

So, in Startup.cs, add:

 public IConfiguration Configuration { get; }
 public Startup(IConfiguration configuration) 
 {
     Configuration = configuration;
 }

and then in ConfigureServices method add what other people have said...

services.AddDbContext<ChromeContext>(options =>                    
    options.UseSqlServer(Configuration.GetConnectionString("DatabaseConnection")));

you also have to ensure that you've got your appsettings.json file created and have a connection strings section similar to this

{
  "ConnectionStrings": {
    "DatabaseConnection": "Server=MyServer;Database=MyDatabase;Persist Security Info=True;User ID=SA;Password=PASSWORD;MultipleActiveResultSets=True;"
  }
}

Of course, you will have to edit that to reflect your configuration.

Things to keep in mind. This was tested using Entity Framework Core 3 in a .Net Standard 2.1 project. I needed to add the nuget packages for: Microsoft.EntityFrameworkCore 3.0.0 Microsoft.EntityFrameworkCore.SqlServer 3.0.0 because that's what I'm using, and that's what is required to get access to the UseSqlServer.

I understand this has been marked as answered but I ran into a bit of a problem when I was working on a project where I have my EF Core Data Access Layer in a .DLL Project separated from the rest of my project, API, Auth and Web and mostly will like my other projects to reference this Data project. And I don't want to want to come into the Data project to change connection strings everytime.

STEP 1: Include this in the OnConfiguring Method

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
      {
           var envName = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
           IConfigurationRoot configuration = new ConfigurationBuilder()
                **.SetBasePath(Path.Combine(Directory.GetCurrentDirectory()))**
                .AddJsonFile("appsettings.json", optional: false)
                .AddJsonFile($"appsettings.{envName}.json", optional: false)
                .Build();
           optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"));
      }

NOTE: .SetBasePath(Path.Combine(Directory.GetCurrentDirectory())) This will negate or invalidate the need to copy the file to a directory as ASP.NET CORE is smart enough to pick the the right file. Also the environment specified will pick right file when the building for Release or Production, assuming the Prod environment file is selected.

STEP 2: Create appsettings.json

{
"ConnectionStrings": {       
  "DefaultConnection": "Server=YOURSERVERNAME; Database=YOURDATABASENAME; Trusted_Connection=True; MultipleActiveResultSets=true"        
} 

}

PLEASE: Referece: Microsoft.Extensions.Configuration

How about passing it as dp injection into that class? in ConfigureServices:

services.Configure<MyOptions>(Configuration);

create class to hold json strings:

public class MyOptions
{
    public MyOptions()
    {

    }
    public string Option1 { get; set; }
    public string Option2 { get; set; }
}    

Add strings to json file:

"option1": "somestring",
"option2": "someothersecretstring"

In classes that need these strings, pass in as constructor:

public class SomeClass
{
 private readonly MyOptions _options;

    public SomeClass(IOptions<MyOptions> options)
    {
        _options = options.Value;           
    }    

 public void UseStrings()
 {
   var option1 = _options.Option1;
    var option2 = _options.Option2;
 //code
 }
}