How to write string array into app config(c#) file and how to read those array values into our .cs file.

probably the easiest way to do this is to enter the value of the key with a delimeter (eg a comma). Use the string's Split function to split the values and there you have your array. Depending on the type of your needed array, you'll need to convert.

config file

<add key="myarray" value="1,5,6,8,9" />

Code behind

string myarrayvalue = ConfigurationManager.AppSettings["myarray"];
string [] str_array = myarrayvalue.Split(new string[]{","}, StringSplitOptions.RemoveEmptyValues);

//if needed as eg. an integer.
int [] myintarray = new int[str_array.Length];
for(int i = 0; i < str_array.Length; i++){
  int output;
  if(int.TryParse(str_array[i], out output){
    myintarray[i] = output;
  }
  else{
    myintarray[i] = -1;
  }
}

This is a pretty basic question, please start out with a good book.