Reading Null Values

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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.

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Nội dung Text: Reading Null Values

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.