Placeholder resolver is not compliant with .net10 changes

[jvmlet]

Describe the bug

.net10 supports setting null and empty string values in json configuration provider.
Placeholder resolver breaks it.

Steps to reproduce

Steps to reproduce the behavior:

 var opt = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json")
            //.AddPlaceholderResolver()  // uncomment this line and test fails
            .Build()
            .GetSection("Kestrel")
            .Get<KestrelServerOptions>();
            
 opt.Limits.MaxResponseBufferSize.Should().BeNull();

"Kestrel": {
    "Limits": {
      "MaxResponseBufferSize": null
    }
  }

Expected behavior

Same behavior as without the placeholder resolver.

Environment

• Steeltoe Version 4.1.0

[bart-vmware]

This is a bug in Steeltoe. While the data in IConfiguration is identical in both cases, the options binder observes different outcomes when it calls PlaceholderConfigurationProvider.TryGet.

Drilling down, the logic here is insufficient:

value = ConfigurationRoot?.GetValue<string>(key);
bool found = value != null;

There is no direct API on IConfigurationRoot to distinguish between "key not found" and "key found, value is null".

Instead, we need to iterate the providers, similar to InternalConfigurationRootExtensions.TryGetConfiguration.

[jvmlet]

@bart-vmware , would you also please add test case for empty string, like

class Model{
 public string CustomRoute{get;set;}
}
var cfg= new ConfigurationBuilder()
            .AddJsonFile("appsettings.json")
            .AddPlaceholderResolver()
            .Build()
            .GetSection("Model")
            .Get<Model>();
            
 cfg.CustomRoute.Should().BeEmpty();

{
"Model":{
 "CustomRoute":""
}
}

Thanks

[bart-vmware]

@bart-vmware , would you also please add test case for empty string

Sure, added.

[bart-vmware]

@jvmlet Now that the fix has been merged, can you try the latest prerelease to verify this meets your needs?

[jvmlet]

Thanks, @bart-vmware , all my tests passed successfully.