.Net Core Strong Typed Configuration Binding for Arrays | Microsoft.Extensions.Configuration

The .Net Core Microsoft.Extensions.Configuration package includes the ConfigurationBinder.Get<>() method which can read a config file and bind name-value entries to a class. This gives you strongly typed configuration:

public class FromTo
{
    public string From { get; init; }
    public string To { get; init; }
}

The method is an extension method for Configuration so you can call it straight off a Configuration instance. In an Asp.Net Startup.cs class, for instance:

services.AddSingleton(
    s=>Configuration
            .GetSection("fromto").Get<FromTo>());

will result in your services collection knowing to provide an FromTo class with the Properties populated from the configuration entries, matched by section:propertyname :

{
  "fromto:from": "[email protected]",
  "fromto:to": "[email protected]"
}

or if you use secrets:

cd <projectdirectory>
dotnet user-secrets init
dotnet user-secrets set fromto:from [email protected]
dotnet user-secrets set fromto:to [email protected]

That works great for the usual primitives - string, numbers, boolean — but what about binding an Array?

public class FromTo
{
    public string From { get; init; }
    public string To[] { get; init; }
}

the From field is still bound but the To field silently fails and results in a null value.

The magic trick is to add a colon-separated number suffix to each setting name:

{
  "fromto:from": "[email protected]",
  "fromto:to:0": "[email protected]",
  "fromto:to:1": "[email protected]",
  "fromto:to:2": "[email protected]",
}

Now Configuration.GetSection("fromto").Get<FromTo>() will successfully bind all 3 rows for "fromto:to into the public string To[] array.