concatenation of two string fields

Hi Community,

 i just try to concatenate two string fields into third field using plugin. But its not working with below code

if (entity.LogicalName == "new_tetest")

try
{
Entity test = new Entity("new_tetest");

String fname = test.GetAttributeValue<String>("new_firstname");
String lname = test.GetAttributeValue<String>("new_lastname");
string concat = fname + lname;
concat = String.Concat(fname, lname);
test["new_fullname"] = concat;
service.Create(test);

}
catch (FaultException<OrganizationServiceFault> ex)
{
throw new InvalidPluginExecutionException("invalidException", ex);
}