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

This problem occurred when the connection string can’t be found.

Probably you have the following code in Startup class:

public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<BenchmarkContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("yourConnectionString name from appsettings.json")));
    }

These methods solve your problem:

1- Instead of Configuration.GetConnectionString("yourConnectionString name from appsettings.json(in develop mode: 'appsettings.Development.json')") just put your connectionstring.

public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<BenchmarkContext>(options =>
  options.UseSqlServer("Data Source=.;Initial Catalog=Benchmark;Persist Security Info=True;User ID=****;Password=****"));
    }

2- If you are going to use the Configuration file add these codes to Startup class:

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

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

Appsettings.json file(in develop mode: ‘appsettings.Development.json’):

{
  "ConnectionStrings": {
    "TestConnection": "Data Source=.;Initial Catalog=Benchmark;Persist Security Info=True;User ID=****;Password=****"
  }
}

After that execute ‘add-migration name’ command in Package Manager Console