.NET Swagger - Members Inherit Documentation From Base Member

Helpers.cs

public static class Helpers
{
	public static XPathNavigator? GetMemberXmlNode(List<XPathDocument> documents, string memberName)
	{
		var path = $"/doc/members/member[@name='{memberName}']";

		foreach (var document in documents)
		{
			var node = document.CreateNavigator().SelectSingleNode(path);
			if (node != null)
			{
				return node;
			}
		}

		return null;
	}

	public static string GetMemberNameForType(Type type)
		=> $"T:{type.FullName}";

	public static string GetMemberNameForMember(MemberInfo member)
		=> $"{(member is PropertyInfo ? "P" : "F")}:{(member.DeclaringType ?? member.ReflectedType).FullName}.{member.Name}";
}

Startup.cs

services.AddSwaggerGen(options => options.SchemaFilter<SwaggerInheritDocSchemaFilter>(options));

SwaggerInheritDocSchemaFilter.cs

 /// <summary>
/// For members that inherit their documentation, this filter will apply the documentation from the base member to the Swagger documentation.
/// Without it, no documentation is applied to members that inherit their documentation.
/// "cref" Attributes are not (yet) supported.
/// </summary>
/// <remarks>
/// Based on https://github.com/domaindrivendev/Swashbuckle.WebApi/issues/1000#issuecomment-1477606163 (accessed 2023-08-08)
/// </remarks>
public class SwaggerInheritDocSchemaFilter : ISchemaFilter
{
	private readonly List<XPathDocument> _documents;
	private readonly HashSet<string> _membersWithInheritedDocs;

	public SwaggerInheritDocSchemaFilter(SwaggerGenOptions options)
	{
		_documents = options.SchemaFilterDescriptors.Where(x => x.Type == typeof(XmlCommentsSchemaFilter))
			.Select(x => x.Arguments.Single())
			.Cast<XPathDocument>()
			.ToList();

		_membersWithInheritedDocs = _documents.SelectMany(document =>
			{
				var inheritedElements = new List<string>();
				foreach (XPathNavigator member in document.CreateNavigator().Select("doc/members/member/inheritdoc"))
				{
					member.MoveToParent();
					inheritedElements.Add(member.GetAttribute("name", ""));
				}

				return inheritedElements;
			})
			.ToHashSet();
	}

	public void Apply(OpenApiSchema schema, SchemaFilterContext context)
	{
		var memberName = Helpers.GetMemberNameForType(context.Type);
		var sources = GetPossibleSources(context.Type);

		if (string.IsNullOrEmpty(schema.Description) && _membersWithInheritedDocs.Contains(memberName))
		{
			foreach (var source in sources)
			{
				var sourceXmlNode = Helpers.GetMemberXmlNode(_documents, Helpers.GetMemberNameForType(source));

				var summaryNode = sourceXmlNode?.SelectSingleNode("summary");
				if (summaryNode != null)
				{
					schema.Description = XmlCommentsTextHelper.Humanize(summaryNode.InnerXml);
				}

				var exampleNode = sourceXmlNode?.SelectSingleNode("example");
				if (exampleNode != null)
				{
					schema.Example = new OpenApiString(XmlCommentsTextHelper.Humanize(exampleNode.InnerXml));
				}
			}
		}

		if (schema.Properties == null)
		{
			return;
		}
		foreach (var entry in schema.Properties)
		{
			var propertyName = entry.Key;
			var property = context.Type.GetProperty(entry.Key, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
			if (property == null)
			{
				continue;
			}

			var propertySchema = entry.Value;
			var propertyMemberName = Helpers.GetMemberNameForMember(property);
			if (!string.IsNullOrEmpty(propertySchema.Description) || !_membersWithInheritedDocs.Contains(propertyMemberName))
			{
				continue;
			}

			foreach (var source in sources)
			{
				var sourceProperty = source.GetProperty(propertyName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
				if (sourceProperty == null)
				{
					continue;
				}

				var sourceXmlNode = Helpers.GetMemberXmlNode(_documents, Helpers.GetMemberNameForMember(sourceProperty));

				var summaryNode = sourceXmlNode?.SelectSingleNode("summary");
				if (summaryNode != null)
				{
					propertySchema.Description = XmlCommentsTextHelper.Humanize(summaryNode.InnerXml);
				}

				var exampleNode = sourceXmlNode?.SelectSingleNode("example");
				if (exampleNode != null)
				{
					propertySchema.Example = new OpenApiString(XmlCommentsTextHelper.Humanize(exampleNode.InnerXml));
				}
			}
		}
	}

	private static List<Type> GetPossibleSources(Type type)
	{
		var targets = type.GetInterfaces().ToList();

		var baseType = type.BaseType;
		while (baseType != typeof(object) && baseType != null)
		{
			targets.Add(baseType);
			baseType = baseType.BaseType;
		}

		targets.Reverse(); // Put most specific types first

		return targets;
	}
}