Enable to acess the session bariable

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C#

C#4.0

i am using webservice for autocomplete that will use in textbox to fetch data from the database but when i try to use session in the below function it will not accessble for me even i have try to store the session variable in global variable but the variable is also not accessable but when i replace "List<string>" with "string[]" it will work fine but then the function will not work so help me to accerss the session variable.

[WebMethod, ScriptMethod]

   public static List<string> GetCountries(string prefixText)
   {
}