When binding to interface collections, config generator shouldn't generate logic for both interface & mapping collection type.

[layomia]

Matching concrete type = actual type to instantiate when binding if needed, e.g.
IList -> List
IDictionary<,> -> Dictionary<,>

---

Using the Geolocation type from @martincostello's repro for #89010, if an interface such as IList<T> or IDictionary<,> is the target binding type, the generator emits logic for both the interfaces and matching config types e.g.

[Fact]
public void Struct_As_Dictionary_Element()
{
    var configuration = TestHelpers.GetConfigurationFromJsonString("""
        {
            "First":
            {
                "Latitude": 3,
                "Longitude": 4,
            }
        }
        """);

    Geolocation obj = configuration.Get<IDictionary<string, Geolocation>>()["First"];
    Assert.Equal(3, obj.Latitude);
    Assert.Equal(4, obj.Longitude);
}

Generated (buggy output per #89010):

public static void BindCore(IConfiguration configuration, ref IDictionary<string, ConfigurationBinderTests.Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key, out ConfigurationBinderTests.Geolocation element)))
        {
            element = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        }
        var temp439 = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        BindCore(section, ref temp439, binderOptions);
        element! = temp439;
        obj[section.Key] = element;
    }
}

public static void BindCore(IConfiguration configuration, ref Dictionary<string, ConfigurationBinderTests.Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key, out ConfigurationBinderTests.Geolocation element)))
        {
            element = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        }
        var temp438 = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        BindCore(section, ref temp438, binderOptions);
        element! = temp438;
        obj[section.Key] = element;
    }
}

We only need one of these methods.

[ghost]

Tagging subscribers to 'size-reduction': @eerhardt, @SamMonoRT, @marek-safar
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Using the Geolocation type from @martincostello's repro for #89010, if a interface such as IList<T> or IDictionary<,> is the target binding type, the generator emits logic for both the interfaces and mapping config types e.g.

[Fact]
public void Struct_As_Dictionary_Element()
{
    var configuration = TestHelpers.GetConfigurationFromJsonString("""
        {
            "First":
            {
                "Latitude": 3,
                "Longitude": 4,
            }
        }
        """);

    Geolocation obj = configuration.Get<IDictionary<string, Geolocation>>()["First"];
    Assert.Equal(3, obj.Latitude);
    Assert.Equal(4, obj.Longitude);
}

Generated (buggy output per #89010):

public static void BindCore(IConfiguration configuration, ref IDictionary<string, ConfigurationBinderTests.Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key, out ConfigurationBinderTests.Geolocation element)))
        {
            element = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        }
        var temp439 = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        BindCore(section, ref temp439, binderOptions);
        element! = temp439;
        obj[section.Key] = element;
    }
}

public static void BindCore(IConfiguration configuration, ref Dictionary<string, ConfigurationBinderTests.Geolocation> obj, BinderOptions? binderOptions)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj));
    }

    foreach (IConfigurationSection section in configuration.GetChildren())
    {
        if (!(obj.TryGetValue(section.Key, out ConfigurationBinderTests.Geolocation element)))
        {
            element = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        }
        var temp438 = InitializeConfigurationBinderTestsGeolocation(section, binderOptions);
        BindCore(section, ref temp438, binderOptions);
        element! = temp438;
        obj[section.Key] = element;
    }
}

We only need one of these methods.

Author:	layomia
Assignees:	layomia
Labels:	enhancement, area-Extensions-Configuration, size-reduction
Milestone:	8.0.0

[ericstj]

I looked at this and it will be more than just a small fix - we don't always generated duplicates - instead we should change what bind methods we produce when we know we will treat the concreate type as it's interface and move the interface cast up above the bindcore call.