Design for inheritance

DesignedForInheritanceTest.cs

using System;
using System.Linq;
using System.Reflection;
using Xunit;

namespace YourLibName.Tests.PatternsAndPractices
{
    public class DesignedForInheritanceTest
    {
        public DesignedForInheritanceTest()
        {
            _assembly = Assembly.Load("YourLibName");
        }

        private readonly Assembly _assembly;

        [Fact]
        public void EveryExportedClass_ShouldBeInheritableOrSealed()
        {
            var classes = _assembly.ExportedTypes.Where(type => type.IsClass).ToList();
            foreach (var type in classes)
            {
                if (type.IsAbstract)
                {
                    continue;
                }

                if (type.IsSealed)
                {
                    continue;
                }

                if (type.GetCustomAttribute<InheritableAttribute>() is object)
                {
                    continue;
                }

                throw new ApplicationException(
                    $"Type '{type}' is public but not abstract, seal it or mark it as [Inheritable].");
            }
        }
    }
}

Examples.cs

using System;

namespace YourLibName
{
    // This class is abstract so it will PASS the test
    public abstract class AbstractClass
    {
    }
    
    // This class is static so it will PASS the test
    public static class StaticClass
    {
    }
    
    // This class is internal so it will PASS the test
    internal class InternalClass
    {
    }
        
    // This class is marked Inheritable so it will PASS the test
    [Inheritable]
    public class InheritableClass
    {
    }
    
    // This class shows no design for inheritance so it will FAIL the test
    public class Foo
    {
    }
}

InheritableAttribute.cs

using System;

namespace YourLibName
{
    /// <summary>Use this attribute to mark non-abstract classes that are designed to be inheritable.</summary>
    [AttributeUsage(AttributeTargets.Class, Inherited = false)]
    internal sealed class InheritableAttribute : Attribute
    {
    }
}