how to add validation on web service that is only verified the first time?I see every verification when called

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C#

WCF

WebForms

C#4.0

webservice

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how to add validation on web service that is only verified the first time and next time called it,it do not need to be verfity ?I see every verification when called online, every time client need to provice username and password,It is not good.Here is my codes,How to correct that?

LocalService.GIISService localTest = new LocalService.GIISService();

           localTest.Credentials = System.Net.CredentialCache.DefaultCredentials;//default credetials

           LocalService.CredentialSoapHeader header = new LocalService.CredentialSoapHeader();//Create SOAP header

           header.UserID = userName;//Set SOAP header user name information

           header.PassWord = userPassword;//Set SOAP header user password information

           localTest.CredentialSoapHeaderValue = header;

           this.Label1.Text = localTest.HelloWorld("ss");