Tài liệu An Example of Using the Get* Methods phần 1 pdf

An Example of Using the Get* Methods Let's take a look at an example that reads the ProductID, ProductName, UnitPrice, UnitsInStock, and Discontinued columns from the Products table using the Get* methods. To figure out which Get* method to use to retrieve a particular SQL Server column type, you use Table 9.4 , shown earlier. For example, the ProductID column is a SQL Server int, and looking up that SQL Server type in Table 9.4 , you can see you use the GetInt32() method to obtain the column value as a C# int. Table 9.5 summarizes the column names, SQL Server types, Get* methods, and C# return types required to retrieve the five columns from the Products table. Table 9.5: Products TABLE COLUMNS, TYPES, AND METHODS COLUMN NAME SQL SERVER COLUMN TYPE GET* METHOD C# RETURN TYPE ProductID int GetInt32() int ProductName nvarchar GetString() string UnitPrice money GetDecimal() decimal UnitsInStock smallint GetInt16() short Discontinued bit GetBoolean() bool Let's assume that you already have a SqlDataReader object named productsSqlDataReader and that it may be used to read the five columns from the Products table. The following while loop uses the Get* methods and returned C# types shown in Table 9.5 to obtain the column values from productsSqlDataReader: while (productsSqlDataReader.Read()) { int productID = productsSqlDataReader.GetInt32(productIDColPos); Console.WriteLine("productID = " + productID); string productName = productsSqlDataReader.GetString(productNameColPos); Console.WriteLine("productName = " + productName); decimal unitPrice = productsSqlDataReader.GetDecimal(unitPriceColPos); Console.WriteLine("unitPrice = " + unitPrice); short unitsInStock = productsSqlDataReader.GetInt16(unitsInStockColPos); Console.WriteLine("unitsInStock = " + unitsInStock); bool discontinued = productsSqlDataReader.GetBoolean(discontinuedColPos); Console.WriteLine("discontinued = " + discontinued); } As you can see, five variables of the appropriate type are created in this while loop, each of which is used to store the result from the Get* method. For example, the productID variable is used to store the ProductID column value, and since ProductID is of the SQL Server int type, the appropriate C# type for the productID variable is int. To get the ProductID column value as a C# int, you call the GetInt32() method. Similarly, the productName variable is a C# string that is used to store the ProductName column value. This column is of the nvarchar SQL Server type, and to get the Product-Name column value, the GetString() method is used. Of course, this code depends on your knowing the type of the database column. If you don't know the type of a column, you can get it using Visual Studio .NET's Server Explorer. For example, Figure 9.1 shows the details of the ProductID column of the Products table. As you can see, ProductID is an int. Figure 9.1: Obtaining the type of a column using Visual Studio .NET's Server Explorer Before closing this section, I will show you how to get the .NET type and database type of a column using C#. You get the .NET type used to represent a column using the GetFieldType() method of your DataReader object. For example: Console.WriteLine("ProductID .NET type = " + productsSqlDataReader.GetFieldType(productIDColPos)); This example displays: ProductID .NET type = System.Int32 As you can see, the System.Int32 .NET type is used to represent the ProductID column. The System.Int32 .NET type corresponds to the C# int type. You can see this type correspondence in Table 9.3 , shown earlier. You can get the database type for a column using the GetDataTypeName() method of your DataReader object. For example: Console.WriteLine("ProductID database type = " + productsSqlDataReader.GetDataTypeName(productIDColPos)); This example displays: ProductID database type = int As you can see, the ProductID column is of the SQL Server int type. Listing 9.2 uses the code examples shown in this section. Listing 9.2: STRONGLYTYPEDCOLUMNVALUES.CS /* StronglyTypedColumnValues.cs illustrates how to read column values as C# types using the Get* methods */ using System; using System.Data; using System.Data.SqlClient; class StronglyTypedColumnValues { public static void Main() { SqlConnection mySqlConnection = new SqlConnection( "server=localhost;database=Northwind;uid=sa;pwd=sa" ); SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText = "SELECT TOP 5 ProductID, ProductName, UnitPrice, " + "UnitsInStock, Discontinued " + "FROM Products " + "ORDER BY ProductID"; mySqlConnection.Open(); SqlDataReader productsSqlDataReader = mySqlCommand.ExecuteReader(); int productIDColPos = productsSqlDataReader.GetOrdinal("ProductID"); int productNameColPos = productsSqlDataReader.GetOrdinal("ProductName"); int unitPriceColPos = productsSqlDataReader.GetOrdinal("UnitPrice"); int unitsInStockColPos = productsSqlDataReader.GetOrdinal("UnitsInStock"); int discontinuedColPos = productsSqlDataReader.GetOrdinal("Discontinued"); // use the GetFieldType() method of the DataReader object // to obtain the .NET type of a column Console.WriteLine("ProductID .NET type = " + productsSqlDataReader.GetFieldType(productIDColPos)); Console.WriteLine("ProductName .NET type = " + productsSqlDataReader.GetFieldType(productNameColPos)); Console.WriteLine("UnitPrice .NET type = " + productsSqlDataReader.GetFieldType(unitPriceColPos)); Console.WriteLine("UnitsInStock .NET type = " + productsSqlDataReader.GetFieldType(unitsInStockColPos)); Console.WriteLine("Discontinued .NET type = " + productsSqlDataReader.GetFieldType(discontinuedColPos)); // use the GetDataTypeName() method of the DataReader object // to obtain the database type of a column Console.WriteLine("ProductID database type = " + productsSqlDataReader.GetDataTypeName(productIDColPos)); Console.WriteLine("ProductName database type = " + productsSqlDataReader.GetDataTypeName(productNameColPos)); Console.WriteLine("UnitPrice database type = " + productsSqlDataReader.GetDataTypeName(unitPriceColPos)); Console.WriteLine("UnitsInStock database type = " + productsSqlDataReader.GetDataTypeName(unitsInStockColPos)); Console.WriteLine("Discontinued database type = " + productsSqlDataReader.GetDataTypeName(discontinuedColPos)); // read the column values using Get* methods that // return specific C# types while (productsSqlDataReader.Read()) { int productID = productsSqlDataReader.GetInt32(productIDColPos); Console.WriteLine("productID = " + productID); string productName = productsSqlDataReader.GetString(productNameColPos); Console.WriteLine("productName = " + productName); decimal unitPrice = productsSqlDataReader.GetDecimal(unitPriceColPos); Console.WriteLine("unitPrice = " + unitPrice); short unitsInStock = productsSqlDataReader.GetInt16(unitsInStockColPos); Console.WriteLine("unitsInStock = " + unitsInStock); bool discontinued = productsSqlDataReader.GetBoolean(discontinuedColPos); Console.WriteLine("discontinued = " + discontinued); } productsSqlDataReader.Close(); mySqlConnection.Close(); } } The output from this program is as follows: ProductID .NET type = System.Int32 ProductName .NET type = System.String UnitPrice .NET type = System.Decimal UnitsInStock .NET type = System.Int16 Discontinued .NET type = System.Boolean ProductID database type = int ProductName database type = nvarchar UnitPrice database type = money UnitsInStock database type = smallint Discontinued database type = bit productID = 1 productName = Chai unitPrice = 18 unitsInStock = 39 discontinued = False productID = 2 productName = Chang unitPrice = 19 unitsInStock = 17 discontinued = False productID = 3 productName = Aniseed Syrup unitPrice = 10 unitsInStock = 13 discontinued = False productID = 4 productName = Chef Anton's Cajun Seasoning unitPrice = 22 unitsInStock = 53 discontinued = False productID = 5 productName = Chef Anton's Gumbo Mix unitPrice = 21.35 unitsInStock = 0 discontinued = True Using the GetSql* Methods to Read Column Values In addition to using the Get* methods to read column values as standard C# types, if you are using SQL Server, you can also use the GetSql* methods. The GetSql* methods return values as Sql* types, which correspond to the actual types used by SQL Server in the database. Note You can see all the GetSql* methods in Table 9.2, shown earlier. . 9 .1 shows the details of the ProductID column of the Products table. As you can see, ProductID is an int. Figure 9 .1: Obtaining the type of a column using. An Example of Using the Get* Methods Let's take a look at an example that reads the ProductID, ProductName, UnitPrice, UnitsInStock, and Discontinued