C# : Adding a new column in datatable and adding value to it

When a query has already populated a DataTable, you can add another column to its schema and then assign a value to that column for each row.

DataSet dataSet = GetResultsDS(sql);

if (dataSet != null && dataSet.Tables.Count > 0)
{
    DataTable table = dataSet.Tables[0];
    table.Columns.Add("DrillDown", typeof(string));

    foreach (DataRow row in table.Rows)
    {
        row["DrillDown"] = "Value for new column";
    }
}

Columns.Add creates the new DrillDown column with the string data type. The loop then sets its value on every existing row. Replace the example text with the value or calculation you need for each row.