c# - Make input to Web Service field optional not required -

i created sample webservice ask user information , return in json form information required. have 2 issues here

1. i don't want make required user enter bot new dates , renewal dates, want make 1 of them required while other 1 optional
2. if both dates, new , renewal, entered user in 1 json not sure how append both lists together.

please let me know if need additional information, here code webservice

    namespace webservicetest     {         /// <summary>         /// summary description webserviceprueba         /// </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 webserviceprueba : system.web.services.webservice         {             private string user { get; set; }             private string userid { get; set; }             private datetime newservicedate { get; set; }             private datetime newservicesto { get; set; }             private datetime renewservicedate { get; set; }             private datetime renewserviceto { get; set; }              [system.web.services.webmethod(description = "get services received", enablesession = true)]             public string getseviceinformation(string name, string userid, datetime newservicedate, datetime newservicesto, datetime renewservicedate, datetime renewserviceto)             {                 user = name;                 userid = userid;                 newservicedate = newservicedate;                 newservicesto = newservicesto;                 renewservicedate = renewservicedate;                 renewserviceto = renewserviceto;                  if (user == "prueba" && userid == "prueba")                 {                     if (renewservicedate == datetime.minvalue || renewserviceto == datetime.minvalue)                     {                         arraylist list = this.participant.getaddedtrctest(newservicedate, newservicesto);                         return jsonconvert.serializeobject(list);                     }                      else if (newservicedate == datetime.minvalue || newservicesto == datetime.minvalue)                     {                         arraylist list = this.participant.getmodifiedtrctest(renewservicedate, renewserviceto);                         return jsonconvert.serializeobject(list);                     }                      else                     {                         arraylist list = this.participant.getaddedtrctest(newservicedate, newservicesto);                         arraylist list2 = this.participant.getmodifiedtrctest(renewservicedate, renewserviceto);                         return jsonconvert.serializeobject(list);                     }                  }                  else if (user != "prueba" && userid == "prueba")                 {                     return "please check username";                 }                  else if (user == "prueba" && userid != "prueba")                 {                     return "please check userid";                 }                  else                 {                     return "please check both username , userid";                 }             }          }     } 

here error when run webservice , don't enter 1 of dates (i.e enter newservicedate , newservicesto not renewservicedate , renewserviceto or other way around

    system.argumentexception: cannot convert  system.datetime.     parameter name: type ---> system.formatexception: string not recognized valid datetime.        @ system.datetimeparse.parse(string s, datetimeformatinfo dtfi, datetimestyles styles)        @ system.convert.todatetime(string value, iformatprovider provider)        @ system.string.system.iconvertible.todatetime(iformatprovider provider)        @ system.convert.changetype(object value, type conversiontype, iformatprovider provider)        @ system.web.services.protocols.scalarformatter.fromstring(string value, type type)        --- end of inner exception stack trace ---        @ system.web.services.protocols.scalarformatter.fromstring(string value, type type)        @ system.web.services.protocols.valuecollectionparameterreader.read(namevaluecollection collection)        @ system.web.services.protocols.htmlformparameterreader.read(httprequest request)        @ system.web.services.protocols.httpserverprotocol.readparameters()        @ system.web.services.protocols.webservicehandler.coreprocessrequest() 

-----------------------------update------------------------

@jianping liu when declare nullable parameters in function no longer able test webservice, `test

the test form available methods primitive types parameters.` here how declaring it

 [system.web.services.webmethod(description = "get services received", enablesession = true)]       public string getseviceinformation(string name, string userid, datetime? newservicedate, datetime? newservicesto, datetime? renewservicedate, datetime? renewserviceto) 

you can use nullable variable 4 datetime

private datetime? newservicedate { get; set; } private datetime? newservicesto { get; set; } private datetime? renewservicedate { get; set; } private datetime? renewserviceto { get; set; } 

and use newservicedate.hasvalue check if contains value.