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

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

> Value cannot be null. Parameter name: connectionString

This is my startup:

namespace MyProject
{
public class Startup
{
public IConfiguration Configuration { get; set; }
public Startup(IConfiguration config)
{
Configuration = config;
}

public void ConfigureServices(IServiceCollection services)
{
services.AddDbContextPool<AppDbContext>(options =>
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
services.AddTransient<IDevRepo, DevRepo>();
services.AddMvc();
services.AddMemoryCache();
services.AddSession();
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
app.UseStatusCodePages();
app.UseStaticFiles();
app.UseMvcWithDefaultRoute();
app.Run(async (context) =>
{
await context.Response.WriteAsync(Configuration["Message"]);
});
}
}
}

program class:

public class Program
{
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.ConfigureAppConfiguration((context, builder) => builder.SetBasePath(context.HostingEnvironment.ContentRootPath)
.AddJsonFile("appsettings.json")
.Build())

.UseStartup<Startup>()
.Build();
}

appsettings.json:

{
"Message": "Hello World",
"ConnectionStrings": {
"DefaultConnection": "Server=(localdb)\\MSSQLLocalDB;Database=NotMyFault;Trusted_Connection=True;MultipleActiveResultSets=true"
}
}

Interestingly if I run the app, it displays "Hello World", but when add migration it cannot find connectionString. Can someone please shed some lights here? Thanks.

Probably, the issue is with your DotNetCliToolReference from the csproj file. If you migrate the project from an older version of asp.net core, the DotNetCliToolReference is not automatically updated.
Update the yourproject.csproj file to use the 2.0.0 version of the CLI as shown in the snippet bellow:

<ItemGroup>

...
<DotNetCliToolReference
Include="Microsoft.EntityFrameworkCore.Tools.DotNet"
Version="2.0.0" />
</ItemGroup>

Rerun, from the project folder, the dotnet command with -v switch to see results

dotnet ef database update -v

Also, recheck your Microsoft.EntityFrameworkCore nuget packages to reference the 2.0.0 version. Remove or update older EF packages. The minimum are:

- Microsoft.EntityFrameworkCore.SqlServer

- Microsoft.EntityFrameworkCore.Design

both 2.0.0 at this moment.

I had a similar problem after trying to use new created project for ASP.NET Core 2.0 Web Api. As far as I found, the cause of the problem was that application settings specified for development environment were not added. I fixed it by updating startup file to the following:

public Startup(IHostingEnvironment env)
{
var builder = new ConfigurationBuilder()
.SetBasePath(env.ContentRootPath)
.AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
.AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
.AddEnvironmentVariables();
this.Configuration = builder.Build();
}

In my case program class looks like the following:

public class Program
{
public static void Main(string[] args)
{
BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>()
.Build();
}

I found my own problem.

I have an AppDbContextFactory class which inherits IDesignTimeDbContextFactory<AppDbContext>. Deleting this class resolves this issue.

I had the same issue, but my solution was a lot simpler. All I did was to change the order of the appsettings.json from:

{
"Message": "Hello World",
"ConnectionStrings": {
"DefaultConnection": "Server=(localdb)\\MSSQLLocalDB;Database=NotMyFault;Trusted_Connection=True;MultipleActiveResultSets=true"
}
}

to:

{
"ConnectionStrings": {
"DefaultConnection": "Server=(localdb)\\MSSQLLocalDB;Database=NotMyFault;Trusted_Connection=True;MultipleActiveResultSets=true"
},
"Message": "Hello World"
}

I have a suspicion that there is a sequence/order of parameters in the appsettings.json file.

I had the same problem, because I was using the default value in **Startup.cs**.
I just edited Configuration property from:

public IConfiguration Configuration { get; }
to:

public IConfiguration Configuration;

and it worked!
If someone say why would be appreciated.

My problem was when I was trying to run App.dll within netcoreapp2.1 folder, but the right folder is netcoreapp2.1\publish\

I had had a similar issue because of the following reasons:

- appsettings.json was not included in the project
- I was running the project from the path which did not contain appsettings.json

I have solved my issue by setting right base path. The problem is the migrations or anything else from different packages uses wrong path to the appsetting.json file. Not sure if it's an official issue.

I have just changed my Startup.cs as follows:

public Startup(IHostingEnvironment env)
{
var builder = new ConfigurationBuilder()
.SetBasePath(env.ContentRootPath)
.AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
.AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true)
.AddEnvironmentVariables();
Configuration = builder.Build();
}

After that you just need to copy your appsettings.json to the right place if it's missing there.

If you have **previously renamed your connection string** in appsettings file and you have omitted to rename it in DesignTimeDbContextFactory class (if you have it in your project) and that is checked by Entity framework, then you may run in this issue.