尝试调用 WCF Webservice 4.0 时出现返回类型无效错误

我正在尝试编写和调用 WCF Web 服务，以下是详细信息：

网页配置：

<add relativeAddress="FetchData.svc" service="WCF.Services.FetchData" />

<service name="WCF.Services.FetchData">
    <endpoint address="" binding="webHttpBinding" bindingConfiguration="" name="FetchData" contract="WCF.Services.FetchData" />
</service>

FetchData 类（示例代码）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.ServiceModel;
using System.ServiceModel.Activation;
using System.Web;
using System.Xml;
using Webservices.Services;
using Data = Webservices.Data;
using System.ServiceModel.Web;
using System.IO;
using System.Net;
using System.ServiceModel.Channels;
using System.Web.UI;
using System.Text;


namespace WCF.Services
{    
    [ServiceContract(Namespace = "urn:WCF.Services.FetchData")]
    public class FetchData
    {
        Data.GetConnect mConnect = new Data.GetConnect();
        private Message RetrievePublishedData(String pub, int number)
        {
            String strOutput = String.Empty; 
            if (!String.IsNullOrEmpty(pub))
            {
                Boolean pubURLExists = mConnect.CheckPubUrlExists(pub);

                if (!pubURLExists)
                {
                    WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.NotFound;
                    return WebOperationContext.Current.CreateTextResponse(String.Format("Requested publication '{0}' is not available.", pub), MimeTypes.TextPlain, Encoding.UTF8);
                }
                using (StringWriter sw = new StringWriterEncoding())
                {
                    using (HtmlTextWriter hw = new HtmlTextWriter(sw))
                    {
                        hw.RenderBeginTag(HtmlTextWriterTag.Html);
                        XmlNode publishedData = mConnect.GetPublishedData(pub, number);
                        hw.RenderEndTag();
                    }
                    return WebOperationContext.Current.CreateTextResponse(sw.ToString(),MimeTypes.TextHTML, Encoding.UTF8);
                }
            }
            return WebOperationContext.Current.CreateTextResponse(strOutput, MimeTypes.TextHTML, Encoding.UTF8);
        }
        [OperationContract]
        [WebGet(UriTemplate = "/published/{number}/{*pub=default}")]
        public Message FetchPublished(String pub, int number)
        {
           return RetrievePublishedData(pub, number);
        }
    }
}

现在，当我尝试浏览网络服务时，出现以下错误：

网络服务 URL -http://localhost:8082/FetchData.svc

Error: 无法加载操作“FetchPublished”，因为它具有 System.ServiceModel.Channels.Message 类型的参数或返回类型，或者具有 MessageContractAttribute 和其他不同类型参数的类型。使用 System.ServiceModel.Channels.Message 或带有 MessageContractAttribute 的类型时，该方法不得使用任何其他类型的参数。

Edit:

namespace WCFWebServices
{
    [ServiceBehavior(AddressFilterMode = AddressFilterMode.Any)]
     [ServiceContract(Namespace = "urn:WCFWebServices.fetchPush")]
        public class FetchData
        {
         [MessageContract]
         public class RetrievePublishedDataInput
         {
             [MessageBodyMember]
             public String pub;
             [MessageBodyMember]
             public String number;
         }
            private Message RetrievePublishedData(RetrievePublishedDataInput input)
            {
                String strOutput = String.Empty;
                String pub = input.pub;
                String number = input.number;
                if (!String.IsNullOrEmpty(pub))
                {
                    Boolean pubURLExists = true;

                    if (!pubURLExists)
                    {
                        WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.NotFound;
                        return WebOperationContext.Current.CreateTextResponse(String.Format("Requested publication '{0}' is not available.", pub), "application/plain; charset=utf-8", Encoding.UTF8);
                    }
                    using (StringWriter sw = new StringWriter())
                    {
                        using (HtmlTextWriter hw = new HtmlTextWriter(sw))
                        {
                            hw.RenderBeginTag(HtmlTextWriterTag.Html);

                            hw.RenderEndTag();
                        }
                        return WebOperationContext.Current.CreateTextResponse(sw.ToString(), "application/html; charset=utf-8", Encoding.UTF8);
                    }
                }
                return WebOperationContext.Current.CreateTextResponse(strOutput, "application/html; charset=utf-8", Encoding.UTF8);
            }
            [OperationContract]
            [WebGet(UriTemplate = "/publishedData/{number}/{pub=default}")]
            public Message FetchPublished(RetrievePublishedDataInput input)
            {
                return RetrievePublishedData(input);
            }
        }      
}

我相信提到的错误是不言自明的。根据the MSDN, using Message类有其自身的限制：

您可以使用 Message 类作为操作的输入参数、操作的返回值或两者。如果 Message 在操作中的任何位置使用，则适用以下限制：

• 该操作不能有任何 out 或 ref 参数。
• 不能有多个输入参数。如果存在该参数，则它必须是 Message 或消息契约类型.
• 返回类型必须是 void、Message 或消息协定类型。

如果您签订了合同，则违反了第二个限制。最简单的解决方法是创建适当的MessageContract:

[MessageContract]
public class RetrievePublishedDataInput
{
  [MessageBodyMember] public string Pub;
  [MessageBodyMember] public int Number;
}

private Message RetrievePublishedData(RetrievePublishedDataInput input)
{
    ....
}