interface IDgnContract
{
    [OperationContract]
    List<string> GetExe(string IP, string func);
    ...
}

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
public class DiagnosticServise : IDgnContract
{
        Tests tests = new Tests();

        public List<string> GetExe(string IP, string func)
        {
            tests.Run(IP, func);
            return tests.Results;
        }
    ...

}