ASP.NET Web API Help Pages 目前還不支援處理主要專案 (通常是 web 專案) 以外的 XML 註解檔 , 所以若專案中有獨立 DTO 專案的設計 , Help Pages 無法順利呈現出內容於說明文件上 , 這裡參考了 StackOverflow 討論串 ( http://goo.gl/Jb1Un3 ) 的替代方案以解決燃眉之急 ; 另外 , Web API Team 也已經接受了開發者的建議 , 準備提供多個 XML Documentation 的解決方案 , 參考：http://aspnetwebstack.codeplex.com/workitem/1720

MultipleXmlDocumentationProvider.cs

public class MultipleXmlDocumentationProvider : IDocumentationProvider, IModelDocumentationProvider
{
    // 多個 XPathNavigator
    private IList<XPathNavigator> _documentNavigators = new List<XPathNavigator>();

    private const string TypeExpression = "/doc/members/member[@name='T:{0}']";
    private const string MethodExpression = "/doc/members/member[@name='M:{0}']";
    private const string PropertyExpression = "/doc/members/member[@name='P:{0}']";
    private const string FieldExpression = "/doc/members/member[@name='F:{0}']";
    private const string ParameterExpression = "param[@name='{0}']";

    /// <summary>
    /// Initializes a new instance of the <see cref="XmlDocumentationProvider"/> class.
    /// </summary>
    /// <param name="documentPaths">The physical path to XML document.</param>
    public MultipleXmlDocumentationProvider(params string[] documentPaths)
    {
        if (documentPaths == null)
        {
            throw new ArgumentNullException("documentPaths");
        }

        // 建構函式採用可變數目引數
        foreach (var documentPath in documentPaths)
        {
            var filePath = Path.GetDirectoryName(documentPath);
            var fileSpec = Path.GetFileName(documentPath);

            // 使用 wildcard 方式，如："Mvc.*.xml" 以取得指涉檔案
            foreach (var file in Directory.GetFiles(filePath, fileSpec))
            {
                XPathDocument xpath = new XPathDocument(file);
                _documentNavigators.Add(xpath.CreateNavigator());
            }
        }
    }

    private XPathNavigator SelectSingleNode(string selectExpression)
    {
        foreach (var navigator in _documentNavigators)
        {
            var propertyNode = navigator.SelectSingleNode(selectExpression);
            if (propertyNode != null)
            {
                return propertyNode;
            }
        }

        return null;
    }

    public string GetDocumentation(HttpControllerDescriptor controllerDescriptor)
    {
        XPathNavigator typeNode = GetTypeNode(controllerDescriptor.ControllerType);
        return GetTagValue(typeNode, "summary");
    }

    public virtual string GetDocumentation(HttpActionDescriptor actionDescriptor)
    {
        XPathNavigator methodNode = GetMethodNode(actionDescriptor);
        return GetTagValue(methodNode, "summary");
    }

    public virtual string GetDocumentation(HttpParameterDescriptor parameterDescriptor)
    {
        ReflectedHttpParameterDescriptor reflectedParameterDescriptor = parameterDescriptor as ReflectedHttpParameterDescriptor;
        if (reflectedParameterDescriptor != null)
        {
            XPathNavigator methodNode = GetMethodNode(reflectedParameterDescriptor.ActionDescriptor);
            if (methodNode != null)
            {
                string parameterName = reflectedParameterDescriptor.ParameterInfo.Name;
                XPathNavigator parameterNode = methodNode.SelectSingleNode(String.Format(CultureInfo.InvariantCulture, ParameterExpression, parameterName));
                if (parameterNode != null)
                {
                    return parameterNode.Value.Trim();
                }
            }
        }

        return null;
    }

    public string GetResponseDocumentation(HttpActionDescriptor actionDescriptor)
    {
        XPathNavigator methodNode = GetMethodNode(actionDescriptor);
        return GetTagValue(methodNode, "returns");
    }

    public string GetDocumentation(MemberInfo member)
    {
        string memberName = String.Format(CultureInfo.InvariantCulture, "{0}.{1}", GetTypeName(member.DeclaringType), member.Name);
        string expression = member.MemberType == MemberTypes.Field ? FieldExpression : PropertyExpression;
        string selectExpression = String.Format(CultureInfo.InvariantCulture, expression, memberName);
        XPathNavigator propertyNode = SelectSingleNode(selectExpression);
        return GetTagValue(propertyNode, "summary");
    }

    public string GetDocumentation(Type type)
    {
        XPathNavigator typeNode = GetTypeNode(type);
        return GetTagValue(typeNode, "summary");
    }

    private XPathNavigator GetMethodNode(HttpActionDescriptor actionDescriptor)
    {
        ReflectedHttpActionDescriptor reflectedActionDescriptor = actionDescriptor as ReflectedHttpActionDescriptor;
        if (reflectedActionDescriptor != null)
        {
            string selectExpression = String.Format(CultureInfo.InvariantCulture, MethodExpression, GetMemberName(reflectedActionDescriptor.MethodInfo));
            return SelectSingleNode(selectExpression);
        }

        return null;
    }

    private static string GetMemberName(MethodInfo method)
    {
        string name = String.Format(CultureInfo.InvariantCulture, "{0}.{1}", GetTypeName(method.DeclaringType), method.Name);
        ParameterInfo[] parameters = method.GetParameters();
        if (parameters.Length != 0)
        {
            string[] parameterTypeNames = parameters.Select(param => GetTypeName(param.ParameterType)).ToArray();
            name += String.Format(CultureInfo.InvariantCulture, "({0})", String.Join(",", parameterTypeNames));
        }

        return name;
    }

    private static string GetTagValue(XPathNavigator parentNode, string tagName)
    {
        if (parentNode != null)
        {
            XPathNavigator node = parentNode.SelectSingleNode(tagName);
            if (node != null)
            {
                return node.Value.Trim();
            }
        }

        return null;
    }

    private XPathNavigator GetTypeNode(Type type)
    {
        string controllerTypeName = GetTypeName(type);
        string selectExpression = String.Format(CultureInfo.InvariantCulture, TypeExpression, controllerTypeName);
        return SelectSingleNode(selectExpression);
    }

    private static string GetTypeName(Type type)
    {
        string name = type.FullName;
        if (type.IsGenericType)
        {
            // Format the generic type name to something like: Generic{System.Int32,System.String}
            Type genericType = type.GetGenericTypeDefinition();
            Type[] genericArguments = type.GetGenericArguments();
            string genericTypeName = genericType.FullName;

            // Trim the generic parameter counts from the name
            genericTypeName = genericTypeName.Substring(0, genericTypeName.IndexOf('`'));
            string[] argumentTypeNames = genericArguments.Select(t => GetTypeName(t)).ToArray();
            name = String.Format(CultureInfo.InvariantCulture, "{0}{{{1}}}", genericTypeName, String.Join(",", argumentTypeNames));
        }
        if (type.IsNested)
        {
            // Changing the nested type name from OuterType+InnerType to OuterType.InnerType to match the XML documentation syntax.
            name = name.Replace("+", ".");
        }

        return name;
    }
}