Article source code: msaccess_sp2.zip
Introduction
Welcome to part two of Access Stored Procedures. Part one described in detail how to create stored procedures in Access using ADO.NET and Visual Basic.NET. Part two will demonstrate how to utilize the stored procedures created in part one by assembling a Database Tier that can be modelled and used in your own applications. This article will describe in detail one implementation of a Database Tier for Visual Basic.NET.
The main purpose of the Database Tier is to provide a gateway to the database via a class module. This class module would act as the glue between the database and the application. There are two main advantages to using a data tier to access your database. You will have the ability to modify your underlying database technology (moving from MS Access to SQL Server for instance) without affecting your application in a major way. You will also be placing a control layer between your application and the database allowing you to ensure that all data is properly "cleansed". The Database Tier in .NET applications would most likely consist of a class module keeping in line with proper object-oriented coding conventions. Earlier versions of Visual Basic would employ a Standard Module to do the job.
Database Tier - Code
It's now time to roll up our sleeves and get dirty with some code. The first thing after adding an empty class declaration file is to pull in the proper .NET Framework libraries listed below.
Imports System
Imports System.Data
Imports System.Data.OleDb
The System Library is standard for most applications, and I make it a habit to include it in almost all my code modules. The System.Data library is necessary for almost all database access applications. The System.Data.OleDb is used specifically for OLEDB Database Providers to which Microsoft Access belongs to. If we were using SQL Server we'd include the custom SQL provider System.Data.SqlClient.
Then next line of code starts the definition of the Class:
Public Class DBTier
Here we've named the Class DBTier and have given it a modifier of Public, thus making it very accessible from other code modules. After the class is defined all properties are declared:
Shared connectionString As String = _
"PROVIDER=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Program " _
& "Files\Microsoft Office\Office10\Samples\Northwind.mdb"
Only one property is declared here as a string variable, connectionString. This variable holds the connection string for the Northwind Access Database. Declaring the variable as Shared defines it as "Class Variable". A class variable is associated with the class, not each object instantiated from the class.
After the connection string declaration you'll find there are three subroutines and one function. The function returns a dataset with a listing of all products. It calls the stored procedure procProductsList, created in part one of this article.
Next you'll find the three subroutines. There is one for each stored procedure; add, update and deletion of products. They're all similarly structured; each with a command, connection and required parameter(s) declared. As a sample, let's dissect the ProductsDeleteItem subroutine. After understanding how this subroutine works the others should be easy to digest.
To start off the routine takes in one parameter, ProductID, which is an Integer representing the Product to be deleted.
Sub ProductsDeleteItem(ByVal ProductID As Integer)
Next, all variables are declared. One for the connection, command and a parameter to be passed into the stored procedure. This parameter is the ProductID to be deleted.
Dim con As OleDbConnection
Dim cmd As OleDbCommand = New OleDbCommand()
Dim paramProductID As New OleDbParameter()
Command and connection objects are initialized:
con = New OleDbConnection(connectionString)
cmd.Connection = con
The paramProductID parameter properties are configured. Then the parameter is added to the command object. In this case the parameter name in the stored procedure is inProductID, it's an integer and the value is set to the ProductID passed into this subroutine.
With paramProductID
.ParameterName = "inProductID"
.OleDbType = OleDbType.Integer
.Size = 4
.Value = ProductID
End With
cmd.Parameters.Add(paramProductID)
The last part actually calls the stored procedure.
cmd.CommandText = "EXECUTE procProductsDeleteItem"
con.Open()
cmd.ExecuteNonQuery()
con.Close()
Notice that the connection object only stays open long enough to carry out the stored procedure and then closes immediately. This reduces any possible contention.
While the DBTier class included in this article clearly describes how to access the stored procedures, it would need some enhancements to become quality production code since no error handling has been added. There may also be the need to further enhance performance here.
The downloaded source code associated with this article includes the DBTier.vb file along with some very basic forms to test the actual implementation of the class.
In conclusion, I hope you have gained at least two things from these articles. One being that stored procedures are alive and well in Microsoft Access, although not without their limitations. The second thing to walk away with here is understanding the need to break down an application's data access into separate classes, subroutines and functions. This makes maintenance and upgrades much easier to implement.
Entire DBTier.vb source code:
Imports System
Imports System.Data
Imports System.Data.OleDb
' Functions and subroutines for executing Stored Procedures in Access.
Public Class DBTier
' Change Data Source to the location of Northwind.mdb on your local
' system.
Shared connectionString As String = _
"PROVIDER=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Program " _
& "Files\Microsoft Office\Office10\Samples\Northwind.mdb"
' This function returns a dataset containing all records in
' the Products Table.
Function ProductsList() As DataSet
Dim con As OleDbConnection
Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim sSQL As String
sSQL = "EXECUTE procProductsList"
con = New OleDbConnection(connectionString)
da = New OleDbDataAdapter(sSQL, con)
ds = New DataSet()
da.Fill(ds, "Products")
Return ds
End Function
' This Function adds one record to the Products table.
Sub ProductsAddItem(ByVal ProductName As String, _
ByVal SupplierID As Integer, ByVal CategoryID As Integer)
Dim con As OleDbConnection
Dim cmd As OleDbCommand = New OleDbCommand()
Dim paramProductName As New OleDbParameter()
Dim paramSupplierID As New OleDbParameter()
Dim paramCategoryID As New OleDbParameter()
con = New OleDbConnection(connectionString)
cmd.Connection = con
With paramProductName
.ParameterName = "inProductName"
.OleDbType = OleDbType.VarChar
.Size = 40
.Value = ProductName
End With
cmd.Parameters.Add(paramProductName)
With paramSupplierID
.ParameterName = "inSupplierID"
.OleDbType = OleDbType.Integer
.Size = 4
.Value = SupplierID
End With
cmd.Parameters.Add(paramSupplierID)
With paramCategoryID
.ParameterName = "inCategoryID"
.OleDbType = OleDbType.Integer
.Size = 4
.Value = CategoryID
End With
cmd.Parameters.Add(paramCategoryID)
cmd.CommandText = "EXECUTE procProductsAddItem"
con.Open()
cmd.ExecuteNonQuery()
con.Close()
End Sub
' This function Updates a specific JobTitle Record with new data.
Sub ProductsUpdateItem(ByVal ProductID As Integer, _
ByVal ProductName As String)
Dim con As OleDbConnection
Dim cmd As OleDbCommand = New OleDbCommand()
Dim paramProductName As New OleDbParameter()
Dim paramProductID As New OleDbParameter()
con = New OleDbConnection(connectionString)
cmd.Connection = con
With paramProductID
.ParameterName = "inProductID"
.OleDbType = OleDbType.Integer
.Size = 4
.Value = ProductID
End With
cmd.Parameters.Add(paramProductID)
With paramProductName
.ParameterName = "inProductName"
.OleDbType = OleDbType.VarChar
.Size = 40
.Value = ProductName
End With
cmd.Parameters.Add(paramProductName)
cmd.CommandText = "EXECUTE procProductsUpdateItem"
con.Open()
cmd.ExecuteNonQuery()
con.Close()
End Sub
' This function deletes one record from the Products table.
Sub ProductsDeleteItem(ByVal ProductID As Integer)
Dim con As OleDbConnection
Dim cmd As OleDbCommand = New OleDbCommand()
Dim paramProductID As New OleDbParameter()
con = New OleDbConnection(connectionString)
cmd.Connection = con
With paramProductID
.ParameterName = "inProductID"
.OleDbType = OleDbType.Integer
.Size = 4
.Value = ProductID
End With
cmd.Parameters.Add(paramProductID)
cmd.CommandText = "EXECUTE procProductsDeleteItem"
con.Open()
cmd.ExecuteNonQuery()
con.Close()
End Sub
End Class
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