c# - The header property/field must be of type SoapHeader or a derived type, or an array of SoapHeader or SoapUnknownHeader -

using system.data; using system.web.services; using system.web.services.protocols;   namespace myservices {     /// <summary>     /// summary description enroll     /// </summary>     [webservice(namespace = "http://tempuri.org/")]     [webservicebinding(conformsto = wsiprofiles.basicprofile1_1)]     [system.componentmodel.toolboxitem(false)]     // allow web service called script, using asp.net ajax, uncomment following line.      // [system.web.script.services.scriptservice]     public class enroll : webservice     {         public credentials credentials = new credentials();          [webmethod(description = "this method call insert public note 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 insert enrollment document database , return relevant document id.", enablesession = false)]         [soapheader("credentials")]         public enrollresponse enrollmentinsert(enrollment enrollmentrequest)         {             return enrollbl.enrollmentinsertbl(enrollmentrequest, this.credentials);         }          [webmethod(description = "this method call retrieve 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 update enrollment document status ready approved , insert public note 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 retrieve history details given document id", enablesession = false)]         [soapheader("credentials")]         public datatable enrollmenthistory(int documentid)         {             return enrollbl.enrollmenthistorybl(documentid, this.credentials);         }          [webmethod(description = "this method call retrieve documents match search criteria", enablesession = false)]         [soapheader("credentials")]         public datatable enrollmentsearch(searchenrollment searchparams)         {             return enrollbl.enrollmentsearchbl(searchparams, this.credentials);         }          [webmethod(description = "this method call retrieves active form types dataset", enablesession = false)]         [soapheader("credentials")]         public dataset formlist()         {             return enrollbl.formlistbl(this.credentials);         }          [webmethod(description = "this method call retrieves document statuses dataset", enablesession = false)]         [soapheader("credentials")]         public dataset statuslist()         {             return enrollbl.statuslistbl(this.credentials);         }     } } 

i created above webservice , added soap header attribute service getting below error

the header property/field enroll.credentials must of type soapheader or derived type, or array of soapheader or soapunknownheader.

description: unhandled exception occurred during execution of current web request. please review stack trace more information error , originated in code.

exception details: system.exception: header property/field enroll.credentials must of type soapheader or derived type, or array of soapheader or soapunknownheader.

i inherit soap header class properties credentials class web service working properly.