Plugin return valid string value first time

 DllImport("SimplePlugin")]
 private static extern string mainSort();
 [DllImport("SimplePlugin")]
 private static extern string testString();
 
 Debug.Log(testString());
 Debug.Log(mainSort());

I wrote a C++ plugin contains a very complicated function mainSort() and return a string while testString() is just return a "Hello" string. When I run it in Unity, the C# script get a valid return value first time while after the first time it works good, and if I change the return value into other type it works good too. And further more, if I call the testString() function before the mainSort() both function work perfectly, so I am totally confused about the reason. Any one meet this problem before? Or any solutions?