Binding to non-null IEnumerable doesn't work

[brockallen]

AB#1277574
When using the binding feature in the config system, for IEnumerable collections that have a default, non-null value, the collection is never assigned into the target model. For example:

For this json:

{
  "config": {
    "data": [ "a", "b" ]
  }
}

and this application code:

 public class Config
    {
        IEnumerable<string> _data = new List<string>();
        public IEnumerable<string> Data
        {
            get
            {
                return _data;
            }
            set
            {
                _data = value;
            }
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            var config = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("test.json")
                .Build();

            var val = config.GetSection("config").Get<Config>();
            Console.WriteLine(val?.Data?.Count());
        }
    }

The output is:

0
Press any key to continue . . .

I'd expect it to print "2".

I'd argue the issue is in BindInstance and explicit handling for IEnumerable should be made (much like the check for IsArray https://github.com/aspnet/Configuration/blob/dev/src/Microsoft.Extensions.Configuration.Binder/ConfigurationBinder.cs#L281).

[sei-jmattson]

I ran into this issue today and came to the same conclusion as @brockallen

[ajcvickers]

We could treat a non-null but empty collection as something that we add to. This way an empty collection would be treated the same as a null collection.

[kevinlo]

Can you handle the non-null and non empty collection case too?

e.g. IEnumerable _data = new[] { "c" };

The Config data can have default value if it does not exist in the JSON and can be overwritten if it exists in the JSON.

I find this problem when I try to bind the IdentityServer4 Client

For version 1.5.2, the allowedGrantTypes is IEnumerable and has default value (it is changed to ICollection in latest dev version). I try to put the client settings in the JSON file but the AllowedGrantTypes can't be overwritten because it is IEnumerable with default value. It works find if the IEnumberable is null or if it is ICollection

    private IEnumerable<string> _allowedGrantTypes = GrantTypes.Implicit; 

[lbogdanov]

@ajcvickers - any updates on when this is going to be fixed? Thank you!

[ajcvickers]

@lbogdanov It's currently in the backlog, which means it's not going to be fixed in an upcoming release. Beyond that, it will depend on prioritization against other work.

[speciesunknown]

The correct behaviour is to overwrite. Not append.
This is how other collections behave, and its generally how a json parser behaves regarding default values on a POCO.

[IanKemp]

For .NET 5.0: https://github.com/dotnet/runtime/blob/master/src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs#L327
For .NET Core 3.1.8: https://github.com/dotnet/extensions/blob/v3.1.8/src/Configuration/Config.Binder/src/ConfigurationBinder.cs#L328

@lbogdanov basically this is never going to be fixed because the team doesn't care enough to do it. Instead of fixing bugs they would rather just keep piling on new features, because bugs are boring and don't get blog posts written about them.