BUILDING THE DATA ACCESS LAYER in .NET framework

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8 BUILDING THE DATA ACCESS LAYER
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NOTE: Microsoft Access does not support multiple SELECT statements in one QueryDef You can still use a DataSet, but it always contains only one DataTable
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Your DAC could have a set of methods that provide each of these objects That gives the business object the option to use whichever object makes the most sense based on the requirements As an example, this section develops the methods that return a DataTable The DAC method to call a stored procedure with parameters is as follows:
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''' ''' ''' ''' <summary> Calls a stored procedure and returns the result </summary> <param name="storedProcedureName">Name of the stored
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''' procedure to execute</param> ''' <param name="arrParam">Parameters required by the stored ''' procedure</param> ''' <returns>DataTable containing the result</returns> ''' <remarks></remarks> Public Shared Function ExecuteDataTable( _ ByVal storedProcedureName As String, _ ByVal ParamArray arrParam() As SqlParameter) As DataTable Dim dt As DataTable ' Open the connection Using cnn As New SqlConnection(MySettingsPTConnectionString) cnnOpen() ' Define the command Dim cmd As New SqlCommand cmdConnection = cnn cmdCommandType = CommandTypeStoredProcedure cmdCommandText = storedProcedureName ' Handle the parameters If arrParam IsNot Nothing Then For Each param As SqlParameter In arrParam cmdParametersAdd(param) Next End If ' Define the data adapter and fill the dataset Dim da As New SqlDataAdapter(cmd) dt = New DataTable daFill(dt) End Using Return dt End Function
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This method is named ExecuteDataTable because it executes a stored procedure and returns a DataTable Notice that this method is public and shared This means that any code that references this component can use these methods without creating an instance of the DAC class
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Building a Data Access Component
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NOTE: To modify this method to take a SQL string instead of a stored procedure name, modify the storedProcedureName parameter to be SQLString Then change the CommandType property of the SqlCommand object in the code to be CommandTypeText, and assign the CommandText property to the SQLString parameter
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The ExecuteDataTable method has two parameters The rst is the name of the stored procedure The second parameter is a set of SqlParameter objects that specify the parameters to pass to the stored procedure Notice the ParamArray keyword This keyword allows the calling code to pass in any number of parameters of the de ned type In this example, the calling code can pass in any number of SqlParameter objects The NET runtime inserts each SqlParameter passed in to this method into the arrParam array (Examples of passing parameters to this method are provided later in this chapter) The code begins by de ning a new Connection object using the connection string identi ed in the PTConnectionString setting (described earlier in this chapter) It then uses the Connection object to open the connection to the database Notice that this code uses the Using statement When you de ne the connection in a Using block, the connection is automatically closed and disposed at the end of the block, even if an error occurs The code then sets up the SqlCommand object The SqlCommand object de nes the stored procedure or SQL text to execute The Connection property associates the command with an open connection The CommandType property de nes whether the command is a stored procedure or SQL statement, and the CommandText property de nes the stored procedure name or SQL statement text If there are any parameters, the code loops through each parameter and adds it to the SqlCommand object s Parameters collection The code then creates a new SqlDataAdapter object for the command and a new DataTable object Creating the SqlDataAdapter object executes the de ned command Finally, the code lls the DataTable object using the SqlDataAdapter object s Fill method and returns the resulting DataTable object
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