c# - ubuntu用mono4跑exe，读取app.config总是异常

ubuntu用mono4跑exe，报读取app.config异常，是怎么回事。用mono3的时候没问题.
异常信息如下：
System.TypeInitializationException: An exception was thrown by the type initializer for Test.Program ---> System.Configuration.ConfigurationErrorsException: Error Initializing the configuration system. ---> System.Configuration.ConfigurationErrorsException: Unrecognized configuration section (/home/lijing/Desktop/iQQ.Net/Test.exe.config line 3)

程序就是一个简单的helloworld

namespace Test
{
    class Program
    {
        protected static readonly NameValueCollection AppSettings = ConfigurationManager.AppSettings;
        private static readonly string rootDir = AppSettings["RootDirPath"] ?? "";

        static void Main(string[] args)
        {
            Console.WriteLine("hello world");
            Console.WriteLine(rootDir);
            Console.ReadKey();
        }
    }
}

mono版本如下：
Mono JIT compiler version 4.0.2 (Stable 4.0.2.5/c99aa0c Wed Jun 24 10:04:37 UTC 2015)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
LLVM: supported, not enabled.
GC: sgen