Hi,

You can use add additional float column and user datacolumn.expression to map with your string column and then calculate average on float column. A sample is here,

DataTable dt = new DataTable();
            DataColumn sal = new DataColumn("Sal", typeof(string));
            dt.Columns.Add(sal);

            DataColumn fsal = new DataColumn("FSal", typeof(float));
            fsal.Expression = "Sal"; 
            dt.Columns.Add(fsal);


            DataRow dr = dt.NewRow();
            dr[0] = "3000";
            dt.Rows.Add(dr);

            DataRow dr2 = dt.NewRow();
            dr2[0] = "6000";
            dt.Rows.Add(dr2);

            float avgsal = (float)dt.Compute("Avg(FSal)", "");

            Console.WriteLine("sal: {0}", avgsal);

Important: It is automatically converting the string to float. So if you have anything other than "Numberical" (e.g. alpahabets, special chars, null etc.) on the string column you'll get runtime exception.

Hope this helps. Let me know how it goes.