SectionContent.cs

using System;
using System.Threading.Tasks;

namespace Microsoft.AspNetCore.Components
{
    public class SectionContent : IComponent, IDisposable
    {
        private SectionRegistry _registry;

        [Parameter] public string Name { get; set; }
        [Parameter] public RenderFragment ChildContent { get; set; }

        public void Attach(RenderHandle renderHandle)
        {
            _registry = SectionRegistry.GetRegistry(renderHandle);
        }

        public Task SetParametersAsync(ParameterView parameters)
        {
            parameters.SetParameterProperties(this);
            _registry.SetContent(Name, ChildContent);
            return Task.CompletedTask;
        }

        public void Dispose()
        {
            if (!string.IsNullOrEmpty(Name))
            {
                // This relies on the assumption that the old SectionContent gets disposed before the
                // new one is added to the output. This won't be the case in all possible scenarios.
                // We should have the registry keep track of which SectionContent is the most recent
                // one to supply new content, and disregard updates from ones that were superseded.
                _registry.SetContent(Name, null);
            }
        }
    }
}

SectionOutlet.cs

using System;
using System.Threading.Tasks;

namespace Microsoft.AspNetCore.Components
{
    public class SectionOutlet : IComponent, IDisposable
    {
        private static RenderFragment EmptyRenderFragment = builder => { };
        private string _subscribedName;
        private SectionRegistry _registry;
        private Action<RenderFragment> _onChangeCallback;

        public void Attach(RenderHandle renderHandle)
        {
            _onChangeCallback = content => renderHandle.Render(content ?? EmptyRenderFragment);
            _registry = SectionRegistry.GetRegistry(renderHandle);
        }

        public Task SetParametersAsync(ParameterView parameters)
        {
            var suppliedName = parameters.GetValueOrDefault<string>("Name");

            if (suppliedName != _subscribedName)
            {
                _registry.Unsubscribe(_subscribedName, _onChangeCallback);
                _registry.Subscribe(suppliedName, _onChangeCallback);
                _subscribedName = suppliedName;
            }

            return Task.CompletedTask;
        }

        public void Dispose()
        {
            _registry?.Unsubscribe(_subscribedName, _onChangeCallback);
        }
    }
}

SectionRegistry.cs

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace Microsoft.AspNetCore.Components
{
    internal class SectionRegistry
    {
        private static ConditionalWeakTable<Dispatcher, SectionRegistry> _registries
            = new ConditionalWeakTable<Dispatcher, SectionRegistry>();

        private Dictionary<string, List<Action<RenderFragment>>> _subscriptions
            = new Dictionary<string, List<Action<RenderFragment>>>();

        public static SectionRegistry GetRegistry(RenderHandle renderHandle)
        {
            return _registries.GetOrCreateValue(renderHandle.Dispatcher);
        }

        public void Subscribe(string name, Action<RenderFragment> callback)
        {
            if (!_subscriptions.TryGetValue(name, out var existingList))
            {
                existingList = new List<Action<RenderFragment>>();
                _subscriptions.Add(name, existingList);
            }

            existingList.Add(callback);
        }

        public void Unsubscribe(string name, Action<RenderFragment> callback)
        {
            if (name != null && _subscriptions.TryGetValue(name, out var existingList))
            {
                existingList.Remove(callback);
            }
        }

        public void SetContent(string name, RenderFragment content)
        {
            if (_subscriptions.TryGetValue(name, out var existingList))
            {
                foreach (var callback in existingList)
                {
                    callback(content);
                }
            }
        }
    }
}