标头属性/字段的类型必须是 SoapHeader 或派生类型，或者是 SoapHeader 或 SoapUnknownH

using System.Data;
using System.Web.Services;
using System.Web.Services.Protocols;

namespace MyServices
{
    /// <summary>
    /// Summary description for Enroll
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class Enroll : WebService
    {
        public Credentials Credentials = new Credentials();
        [WebMethod(Description = "This method call will insert a public note with the given comments", EnableSession = false)]
        [SoapHeader("Credentials")]
        public bool ActivityInsert(int docId, string activityType, string comments, string userNameOn247, string userNameEt)
        {
            return EnrollBl.ActivityInsertBl(docId, activityType, ref comments, userNameOn247, userNameEt, this.Credentials);
        }
        [WebMethod(Description = "This method call will insert the enrollment document into the database and return the relevant document id.", EnableSession = false)]
        [SoapHeader("Credentials")]
        public EnrollResponse EnrollmentInsert(Enrollment enrollmentRequest)
        {
            return EnrollBl.EnrollmentInsertBl(enrollmentRequest, this.Credentials);
        }
        [WebMethod(Description = "This method call will retrieve the pdf file based on document id", EnableSession = false)]
        [SoapHeader("Credentials")]
        public object EnrollmentFile(int documentId)
        {
            return EnrollBl.EnrollmentFileBl(documentId, this.Credentials);
        }
        [WebMethod(Description = "This method call will update the enrollment document status from Ready to Approved and also insert a public note with the given comments", EnableSession = false)]
        [SoapHeader("Credentials")]
        public bool EnrollmentUpdate(int docId, string activityType, string comments, string userNameEt, string userNameOn247)
        {
            return EnrollBl.EnrollmentUpdateBl(docId, activityType, ref comments, userNameOn247, userNameEt, this.Credentials);
        }
        [WebMethod(Description = "This method call will retrieve the history details for the given document id", EnableSession = false)]
        [SoapHeader("Credentials")]
        public DataTable EnrollmentHistory(int documentId)
        {
            return EnrollBl.EnrollmentHistoryBl(documentId, this.Credentials);
        }
        [WebMethod(Description = "This method call will retrieve all the documents that match the search criteria", EnableSession = false)]
        [SoapHeader("Credentials")]
        public DataTable EnrollmentSearch(SearchEnrollment searchParams)
        {
            return EnrollBl.EnrollmentSearchBl(searchParams, this.Credentials);
        }
        [WebMethod(Description = "This method call retrieves all the active form types as dataset", EnableSession = false)]
        [SoapHeader("Credentials")]
        public DataSet FormList()
        {
            return EnrollBl.FormListBl(this.Credentials);
        }
        [WebMethod(Description = "This method call retrieves all document statuses as dataset", EnableSession = false)]
        [SoapHeader("Credentials")]
        public DataSet StatusList()
        {
            return EnrollBl.StatusListBl(this.Credentials);
        }
    }
}

创建了上面的网络服务，并向服务添加了肥皂标头属性，我收到以下错误

标头属性/字段 Enroll.Credentials 的类型必须是 SoapHeader 或派生类型，或者是 SoapHeader 或 SoapUnknownHead 的数组。

说明：执行当前 Web 请求期间发生未经处理的异常。请查看堆栈跟踪，了解有关错误及其在代码中起源位置的详细信息。

异常详细信息：System.Exception：标头属性/字段 Enroll.Credentials必须为 SoapHeader 或派生类型，或者 SoapHeader 或 SoapUnknownHead 数组。