How to add column in SQL server table

ALTER TABLE <table_name>
(
  ADD <column name=""> <optional constraint>
  [, ... ]
)

using (SqlConnection connection = new SqlConnection("Your connection string"))
            {
                connection.Open();
                using (SqlCommand command = new SqlCommand())
                {
                    command.CommandText = string.Format("ALTER TABLE TABLE_NAME ADD {0} {1}", "[txtColumnName.Text]", "[cmbColumnType.SelectedValue]");
                    command.Connection = connection;
                    command.ExecuteNonQuery();
                }
            }