Best way to check for string in comma-delimited list with .NET?

Question

I\'m reading a comma-delimited list of strings from a config file. I need to check whether another string is in that list. For example:

\"apple,banana,cheese\"

Answer 1

                 public bool GetIsExistSubstring(string mainstring, string substring)
    {        
        bool Isexist=false;
        int i = -1;
        mainstring = string.Format(",{0},", mainstring);//so it will be like  ,1,2,3,
        substring = string.Format(",{0},", substring);//so it will be like  ,1,
        i=mainstring.IndexOf(substring);

        if(i!=-1)
        {
            Isexist = true;
        }
        return Isexist;
    }