Different between String type and GH_String type

I'm wondering what is the difference between String type data with GH_String type data. I tried to used it in C# with the same script, but I get completely different result.

My script worked in String type but not in GH_String. Besides, i need to declare the variable in GH_String somehow.

Well, it's in the if condition:

List<GH_String> inputlist = new List<GH_String>();

for (int i = 0; i < inputlist.Count; i += 5)
{ if (!inputlist.GetRange(0,i).Contains(inputlist[i])) {...}}

In GH, for example I set i=apple; seems like it doesn't want to read the data "apple", but the item only "inputlist[i]".