Could not figure out "size is invalid" on decimal argument

Question

There is a (SOAP) webservice that I am calling. If I use SOAP-UI tool with the same values, it works good. However, when I call through C# code, it is giving me this error. I do not have a proper brought up in ASP .Net world.

if ( serviceFee > 0 )
            {
                surchargeItemsReq SurchargeItemsReqObject = new attachSurchargeItemsRequest()
                {
                    aID = this._applicationId,
                    tID = token,
                    provSurcharge = ((decimal)1.8)
                };

                try
                {
                    this.WriteLog("Invoking Webservice SurchargeItems with ServiceFee");
                    surchargeItemsResponse SurchargeItemsRespObject = ws.surchargeItems(SurchargeItemsReqObject);
                }

The decimal variable had 1.8 as value in it. When I was using the variable itself, I get the same issue. So I literally put (decimal)1.8 in there. Same issue. Please note that in SoapUI tool, request with same values literally put in, works good.

Exception I am getting is this.

System.Web.Services.Protocols.SoapHeaderException was caught
  HResult=-2146233087
  Message=provSurcharge size is invalid
  Source=System.Web.Services
  StackTrace:
       at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
       at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
       at com.sac.commerce.providers.KanPayService.ServerPaymentPortalService.surchargeItems(surchargeItemsRequest 
  InnerException: 

Here is the request object (in visual studio generated code)

public partial class surchargeItemsReq {
        private decimal provSurchargeField;
        private bool provSurchargeFieldSpecified;
        
        public decimal provSurcharge {
            get {
                return this.provSurchargeField;
            }
            set {
                this.provSurchargeField = value;
            }
        }

Answer 1

@HansKilian commented saying to set the provSurchargeSpecified boolean flag value. After setting it to true, it worked. Who would have thought this?

if ( serviceFee > 0 )
{
    surchargeItemsRequest SurchargeItemsReqObject = 
        new surchargeItemsRequest()
        {
           provSurcharge = serviceFee,
           provSurchargeSpecified = true
        };

I wonder why for only some fields (bool or decimal types) have this extra boolean fieldspecified property code-generated. At first I thought I could find something in the WSDL-XSD. There are other minOccurs="0" type fields too that do not have this kind of code generated.

<xs:element minOccurs="0" name="provSurcharge" type="xs:decimal"/>