Đang chuẩn bị liên kết để tải về tài liệu:
Reading Null Values

Không đóng trình duyệt đến khi xuất hiện nút TẢI XUỐNG Tải xuống

Reading Null Values As you learned in Chapter 2, a column defined as null can store a null value. A null value indicates that the column value is unknown. | Reading Null Values As you learned in Chapter 2 a column defined as null can store a null value. A null value indicates that the column value is unknown. A standard C type cannot store a null value but a Sql type can. Let s consider an example of reading a null value from the database. Say you ve performed a SELECT statement that retrieves the UnitPrice column for a row from the Products table and that the UnitPrice column contains a null value. If you try to store that null value in a standard C type such as a decimal using the following code decimal unitPrice productsSqlDataReader.GetDecimal unitPriceColPos then you ll get the following exception System.Data.SqlTypes.SqlNullValueException You ll also get this exception if you try to store the null value in an object as shown in the following example object unitPriceObj productsSqlDataReader UnitPrice You can check if a column contains a null value using the IsDBNull method of a DataReader object. This method returns a Boolean true or false value that indicates whether the column value is null. You can then use that Boolean result to decide what to do. For example if productsSqlDataReader.IsDBNull unitPriceColPos Console.WriteLine UnitPrice column contains a null value else unitPrice productsSqlDataReader.GetDecimal unitPriceColPos Because productsSqlDataReader.IsDBNull unitPriceColPos returns true this example displays UnitPrice column contains a null value As mentioned a Sql type can store a null value. A null value is stored as Null. For example SqlMoney unitPrice productsSqlDataReader.GetSqlMoney unitPriceColPos Console.WriteLine unitPrice unitPrice This example displays unitPrice Null Each of the Sql types also has a Boolean property named IsNull that is true when the Sql object contains a null value. For example Console.WriteLine unitPrice.IsNull unitPrice.IsNull This example displays unitPrice.IsNull True True is displayed because unitPrice contains .