public class EncryptMessage : SoapExtension
{
public int num=10;
....
}

Soap extension on web method:

public class Service1 : System.Web.Services.WebService
{
/*in this place I want call some methods, which change variable num in class EncryptMessage,
before than is soap extension used on web method .. it is possible ??If yes, how can I change variable in class EncryptMessage*/

int num2=5;
someMethods(num2); // this methods change variable num in 
class EncryptMessage

public const string k = "something";
[WebMethod]
[EncryptMessage(SetKey =k)]
public string test2()
{
return "ok";
}
}