GridView positive and negative way sorting

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C#

I need to sort gridview with negative and positive int values. Presently the gridview default sorting working as the negative sign was not considering.

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private string getSortDirectionString(string Sort)
   {
       if (ViewState[Sort] == null)
       {
           ViewState[Sort] = "ASC";
       }
       else
       {
           if (ViewState[Sort].ToString() == "ASC")
           {
               ViewState[Sort] = "DESC";
               return ViewState[Sort].ToString();
           }
           if (ViewState[Sort].ToString() == "DESC")
           {
               ViewState[Sort] = "ASC";
               return ViewState[Sort].ToString();
           }
       }
       return ViewState[Sort].ToString();
   }