Naive behaviour for ConfigurationBinder

[no1melman]

So these lines:

runtime/src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs

Lines 354 to 378 in c6d0daf

	if (configValue != null && TryConvertValue(type, configValue, section.Path, out convertedValue, out error))
	{
	if (error != null)
	{
	throw error;
	}
	// Leaf nodes are always reinitialized
	return convertedValue;
	}
	if (config != null && config.GetChildren().Any())
	{
	// If we don't have an instance, try to create one
	if (instance == null)
	{
	// We are already done if binding to a new collection instance worked
	instance = AttemptBindToCollectionInterfaces(type, config, options);
	if (instance != null)
	{
	return instance;
	}
	instance = CreateInstance(type);
	}

I was trying to create a custom type provider to handle Option<_> in F#.

Options replace null - essentially. However, the code on Line 354 says

configValue != null && TryConvertValue(...)

Which means now, it will just switch to trying to create a default type using an empty constructor (Activator.CreateInstance), referenced on line 377 instance = CreateInstance(type); which does:

            if (!type.IsValueType)
            {
                bool hasDefaultConstructor = type.GetConstructors(DeclaredOnlyLookup).Any(ctor => ctor.IsPublic && ctor.GetParameters().Length == 0);
                if (!hasDefaultConstructor)
                {
                    throw new InvalidOperationException(SR.Format(SR.Error_MissingParameterlessConstructor, type));
                }
            }

Option doesn't have a default constructor... so now I can't handle a null configValue and convert to a type without an empty constructor.

Which seems flawed to me.

Is there a reason behind this logic, or just a missed use case?