Returning Strongly Typed Column Values

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Returning Strongly Typed Column Values

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Returning Strongly Typed Column Values Up to this point, you've retrieved column values from a DataReader only as generic objects of the System.Object class

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  1. Returning Strongly Typed Column Values Up to this point, you've retrieved column values from a DataReader only as generic objects of the System.Object class (such objects are often referred to as being of the C# object type). Note All classes in C# are derived from the System.Object class. I'll rewrite the while loop shown in the example in the previous section to show how you store column values as objects of the System.Object class: while (productsSqlDataReader.Read()) { object productID = productsSqlDataReader[productIDColPos]; object productName = productsSqlDataReader[productNameColPos]; object unitPrice = productsSqlDataReader[unitPriceColPos]; object unitsInStock = productsSqlDataReader[unitsInStockColPos]; object discontinued = productsSqlDataReader[discontinuedColPos]; Console.WriteLine("productID = " + productID); Console.WriteLine("productName = " + productName); Console.WriteLine("unitPrice = " + unitPrice); Console.WriteLine("unitsInStock = " + unitsInStock); Console.WriteLine("discontinued = " + discontinued); } This code results in the same output as Listing 9.1. All I did in Listing 9.1 is to explicitly show that a DataReader returns a column value as an object of the System.Object class by default. When an object of the System.Object class is displayed by the Console.WriteLine() method, the object is first implicitly converted to a string and then displayed. That's fine for just displaying the column values, but what if you want to perform some kind of calculation with a value? To do that, you must first cast the value to a specific type. The following example casts the unitPrice object to a decimal and then multiplies it by 1.2:
  2. decimal newUnitPrice = (decimal) unitPrice * 1.2m; Note You add an m to the end of a literal number to indicate it is of the decimal type. Casting an object to a specific type works, but it's not very elegant. It also goes against the one of the main benefits of a modern programming language: use of strong typing. Strongly typing means that you pick the type of a variable or object when declaring it. The main benefit of strong typing is that you're less likely to have runtime errors in your programs that are caused by using the wrong type. This is because the compiler checks your code to make sure the context of the type is correct. The bottom line is that you should endeavor to make all your variables and objects of the appropriate type to begin with, and use casting only when you have no other choice. In this case, you have a choice: instead of casting, you can use one of the DataReader object's Get* methods to return a column value in an appropriate type. Note I use the asterisk in Get* to indicate there are many methods that start with Get. You can see all the Get* methods in Table 9.2, shown earlier. For example, one of the Get* methods is GetInt32(), which returns a column value as an int value. The following code shows the use of the GetInt32() method to obtain the column value for the ProductID column as an int: int productID = productsSqlDataReader.GetInt32(productIDColPos); As you can see, you pass the ordinal of the column that has the value you want to obtain to the Get* method. You saw how to get a column's ordinal value in the previous section.
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