binding options classes with polymorphic lists to config sections

[dazinator]

I've recently handled serialising and deserialising polymorphic lists with system.text.json, and described an issue I had to workaround here: #56297

This above issue covers a case where I want to use SJT to directly serialise / deserialise an instance of my class with a polymorphic list which is good.

However suppose I wanted to use my class with a polymorphic list, with the IOptions pattern. In this case system.text.json converter is now redundant, as ConfigurationBuilder.AddJson essentially reads the json into key value pairs in the IConfiguration - it doesn't deserialise the json into instances of concrete types. This is ofcourse how its meant to work, but it does then mean that if I want an instance of an options class containing a polymorphic list (where each item in the list has some type descriminator specified as a config value) I need a way to take control of how the object instance is materialised from the IConfiguration values. For example, when my options instance is created, I want to be able to have some logic inspect the key value pairs in IConfiguration for it's polymorphic list property, and then for each item, check its discriminator value, and then new() up (or bind) the appropriate derived type to add to the polymorphic list. Is there something on the roadmap that would enable me to do this?

Tagging subscribers to this area: @eiriktsarpalis, @layomia
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Issue Details

---

I've recently handled serialising and deserialising polymorphic lists with system.text.json, and described an issue I had to workaround here: #56297

This covers a case where I want to use SJT to directly serialise / deserialise an instance of my class with a polymorphic list which is good.

However suppose I wanted to use this class with the IOptions pattern. In this case system.text.json converter is now redundant, as ConfigurationBuilder.AddJson essentially reads the json into key value pairs in the IConfiguration - it doesn't deserialise the json into instances of concrete types. This is ofcourse how its meant to work, but it does then mean that if I want an instance of an options class containing a polymorphic list (where each item in the list has some type descriminator property) I need a way to take control of how the object instance is materialised from the IConfiguration values. For example, when I inject my IOptions instance I want to be able to have some logic inspect the key value pairs in IConfiguration for the polymorphic list property, and then for each item, check its descriminator value, and then new() up (or bind) the appropriate derived type to add to the list. Is there something on the roadmap that would enable me to do this?

Author:	dazinator
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

Tagging subscribers to this area: @maryamariyan
See info in area-owners.md if you want to be subscribed.

Issue Details

---

I've recently handled serialising and deserialising polymorphic lists with system.text.json, and described an issue I had to workaround here: #56297

This above issue covers a case where I want to use SJT to directly serialise / deserialise an instance of my class with a polymorphic list which is good.

However suppose I wanted to use my class with a polymorphic list, with the IOptions pattern. In this case system.text.json converter is now redundant, as ConfigurationBuilder.AddJson essentially reads the json into key value pairs in the IConfiguration - it doesn't deserialise the json into instances of concrete types. This is ofcourse how its meant to work, but it does then mean that if I want an instance of an options class containing a polymorphic list (where each item in the list has some type descriminator property) I need a way to take control of how the object instance is materialised from the IConfiguration values. For example, when my options instance is created, I want to be able to have some logic inspect the key value pairs in IConfiguration for it's polymorphic list property, and then for each item, check its discriminator value, and then new() up (or bind) the appropriate derived type to add to the polymorphic list. Is there something on the roadmap that would enable me to do this?

Author:	dazinator
Assignees:	-
Labels:	area-Extensions-Options, untriaged
Milestone:	-

[williamb1024]

I have a similar issue. I my scenario, I need to bind a settings object containing an ICollection<X509Certificate2>. Registering a custom TypeConverter is generally straight forward, including the ability to register a TypeConverter for the ICollection<X509Certificate2> type.

Ultimately, the registered TypeConverter receives a single string (the value of the configuration section) and is able to return the appropriate object. This works, but isn't very flexible. An entire syntax has to be created for all of the possible certificate sources, for example, a certificate store or a certificate file. If there a multiple certificates, those all be specified in the same string.

If it would be possible to modify BindInstance to attempt to pass the TypeConverter and the IConfigurationSection before trying to convert from String, the TypeConverter would have the opportunity to read the IConfigurationSection content in any manner it desired. This would allow for multiple keys, or even arrays, to be read by the TypeConverter and used for the binding process.

I believe it would be a relatively unobtrusive change, as I suspect most TypeConverters would simply ignore the IConfigurationSection type when passed to CanConvertFrom.

[KalleOlaviNiemitalo]

If you make ConfigurationBinder pass the IConfigurationSection to the TypeConverter, please consider also making the IConfigurationRoot available to the TypeConverter. At the TypeConverter side, this could be done via the System.ComponentModel.ITypeDescriptorContext? context parameter of TypeConverter.ConvertFrom; ITypeDescriptorContext is derived from IServiceProvider, so the converter could call context?.GetService(typeof(IConfigurationRoot)). At the ConfigurationBinder side, the IConfigurationRoot could perhaps be added as a property in BinderOptions.

In my scenario, some of the classes that I want to bind to configuration have database connection string properties, and I want to have only the name of the connection string as a configuration value, and read the provider name and connection string from the "ConnectionStrings" section of the configuration root during the conversion. (There is ConfigurationExtensions.GetConnectionString for reading the connection string; there isn't a similar method for reading the provider name, but Microsoft.Extensions.Configuration.EnvironmentVariables defines a convention.)

Or more generally, let the caller of ConfigurationBinder provide an IServiceProvider and make the IServiceProvider implementation of the ITypeDescriptorContext forward the calls to that.

[dazinator]

Is there any new on this?
I am not sure whether TypeCoverter is the best fit for this.
I am exploring an approach that uses IConfigureOptions
e.g the following class could be generecised to take the TOptions and the config section path and then re-used.

public class MyOptionsSetup : IConfigureOptions<MyOptions>
{
    private readonly IConfiguration _configuration;

    public MyOptionsSetup(IConfiguration configuration)
    {
        _configuration = configuration;
    }

    public void Configure(MyOptions options)
    {
        // Custom logic to populate options.MyDictionary
        // Similar to the PolymorphicJsonConverter.Convert method
        options.MyDictionary = ConvertToPolymorphicDictionary(_configuration.GetSection("MyDictionary"));
    }

    private Dictionary<string, SomeBase> ConvertToPolymorphicDictionary(IConfigurationSection section)
    {
        var result = new Dictionary<string, SomeBase>();
        foreach (var child in section.GetChildren())
        {
            var type = GetTypeFromConfiguration(child);
            var instance = (SomeBase)Activator.CreateInstance(type);
            JsonConvert.PopulateObject(child.Value, instance);
            result.Add(child.Key, instance);
        }
        return result;
    }

    private Type GetTypeFromConfiguration(IConfigurationSection section)
    {
        var typeName = section["type"];
        return Type.GetType(typeName);
    }
}