How To Avoid the Boxing Penalty When You Use the DataReader in Visual Basic .NET

This article describes how to use the Getxxx methods (such as GetChar, GetDouble, and GetInt32) to avoid the boxing penalty when you use the DataReader object.

Description of the Technique

When you use the Item property to read columns from a DataReader, the values are boxed and then unboxed. If the values are repeatedly boxed and unboxed, the heap quickly fills and increases the frequency of garbage collections. This also slightly impacts performance because Microsoft Visual Studio .NET converts and copies data more than necessary.

NOTE: Boxing means that the data is copied onto the heap as System.Object. When you cast to a specific data type, the values are unboxed and copied onto your variables on the stack or into another object.

To avoid the boxing penalty, use the Getxxx methods (such as GetChar, GetDouble, and GetInt32) that return the data as a simple data type instead of as System.Object.

Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that you need:

• Microsoft Windows 2000 Professional, Windows 2000 Server, Windows 2000 Advanced Server, or Windows NT 4.0 Server
• Microsoft Visual Studio .NET

This article assumes that you are familiar with the following topics:

• Visual Studio .NET
• ADO.NET fundamentals and syntax

Create Project and Add Code

1. Start Visual Studio .NET.
2. Create a new Windows Application in Visual Basic .NET. By default, Form1 is added to the project.
3. Make sure that your project contains a reference to the System.Data namespace, and add a reference to this namespace if it does not.
4. Add a Button control to Form1. Change the Name property of the button to btnTest, and change the Text property to Test.
5. Use the Imports statement on the System and System.Data namespaces so that you do not have to qualify declarations in those namespaces later in your code. Add this code to the "General Declarations" section of Form1:

   Imports System
   Imports System.Data
   Imports System.Data.OleDb
   Imports System.Data.SqlClient
   					

6. Copy and paste the following code in the btnTest_Click event:

       Dim myConnString as String = _
           "User ID=sa;password=sa;Initial Catalog=northwind;Data Source=myServer"
       Dim mySelectQuery As String = _
           "Select * From Customers"
       Dim con As New SqlConnection(myConnString)
       Dim myCommand As New SqlCommand(mySelectQuery, con)
           
       con.Open()
       Dim myReader As SqlDataReader = myCommand.ExecuteReader()
       Dim str1 As String
   
       While myReader.Read()
           'This code uses the GetString method.
           'str1 = str1 & myReader.GetString(0) & ", "
           'This code uses the Item method.
           str1 = str1 & myReader.Item(0) & ", "
       End While
       
       MessageBox.Show(str1)
   
       myReader.Close()
       con.Close()
   					

7. Modify the connection string (myConnString) as appropriate for your environment.
8. Save your project. On the Debug menu, click Start to run your project.
9. Click Test. Notice that the query uses one of the methods (GetString or Item) to return data, and the message boxes display this data.
10. To compare the difference in performance or to time the code, use the QueryPerformanceCounter function to time application code. For more information about QueryPerformanceCounter, see the REFERENCES section.

Pitfalls

The disadvantage of using the Getxxx method is that you must check for NULL before you access the field. To check for NULL, use the IsDBNull method.

For more information about ADO.NET objects and syntax, see the following topic in the Microsoft .NET Framework Software Development Kit (SDK) documentation or MSDN Online: For additional information about how to use QueryPerformanceCounter to time code in Visual Basic .NET, click the article number below to view the article in the Microsoft Knowledge Base: