Tài liệu Reading Null Values doc

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.. Let's consider an example of

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 Null