Get the Sum of a Particular DataTable Column

Use the DataTable.Compute method: 

    DataTable table;
    table = dataSet.Tables["Orders"];

    // Declare an object variable. 
    object sumObject;
    sumObject = table.Compute("Sum(Total)", "EmpID = 5");

where "EmpID = 5" is a filter expression. 

Use empty string if filter expression is not needed.