Value cannot be null. Parameter name: connectionString appsettings.json in starter

First of all, the

 "Data": {
"ConnectionStrings": {
  "DefaultConnection": "Data Source=server;Initial Catalog=dbase;Trusted_Connection=True;MultipleActiveResultSets=true"},
}

Is slightly different from the structure you get when you add a "Asp.NET Configuration File" in Visual Studio. When you do that you get

"ConnectionStrings": {
  "DefaultConnection": "Data Source=server;Initial Catalog=dbase;Trusted_Connection=True;MultipleActiveResultSets=true"},

without the "Data" JavaScript Object. So that's why the extension method isn't working. It expects this structure. You can use this structure (the one with "Data") anyway and get your connection string like so:

var connectionString = Configuration["Data:ConnectionStrings:DefaultConnection"];

Notice that you are navigating through the JavaScript object tree using : instead of .. That's due to some cross-platform issues with using the ..

If you edit out the "Data":{} you can do this :

var connectionString = Configuration["ConnectionStrings:DefaultConnection"];

Now the extension method will work. Underneath the Microsoft extensions it is the same thing as the code above.

var config2 = Configuration.GetConnectionString("DefaultConnection");

I was missing the letter 's' after the ConnectionString property name in the appsettings.json when using Configuration.GetConnectionString("name")

If you want to copy

"ConnectionStrings ": {
  "SpyStore": "Server=(localdb)\\mssqllocaldb;Database=SpyStore;Trusted_Connection=True;MultipleActiveResultSets=true;"
}

The method wording GetConnectionString confused me, I hovered over it and oh be hold it was looking for ConnectionStrings property name instead of ConnectionString

DefaultConnection is the inner object in the json structure and it is the child of Data object.

So if you want to be exact with your config file you can use

var connStr = Configuration.GetSection("Data")
                           .GetSection("DefaultConnection")["ConnectionString"];