Building a Data Access Component in Visual Studio .NET

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Use application settings to de ne your connection string
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NOTE: Because the appconfig file is deployed and viewable, take precautions to avoid revealing sensitive information in the connection string, such as passwords or server paths Consider using encryption to encrypt the connection string in the appconfig file
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8 BUILDING THE DATA ACCESS LAYER
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Even though Visual Studio allows you to create an appcon g le for a Class Library project, the NET Framework does not provide a con guration le model for class libraries This means that when you run the application, the appcon g le for any class library is ignored You have to manually copy any settings (including the connection string) from the app con g le de ned for any Class Library project into the appcon g le for the application, which is de ned in the startup Windows Application project
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NOTE: When you build the application, the appconfig file for the startup Windows Application project is renamed with the name of the Windows Application project with a config extension This is the only configuration file the application accesses
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The appcon g le for your Class Library project may appear to work This is because the setting you de ned in the Project Designer is used as the default Using application settings to store the connection string makes it easy for the application to connect to a database Any code in the DAC can reference the connection using MySettingsPTConnectionString
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For the Purchase Tracker sample application: Copy the connection string for the PurchaseTracker database from Server Explorer to the Clipboard, as detailed in this section Create a setting for the connection string in the data access component Class Library project (PTDAC), as detailed in this section Name the setting PTConnectionString Paste the connection string from the Clipboard to the Value column of the new setting Open the appconfig file for the data access component Class Library project (PTDAC) Copy the entire connectionStrings tag, and paste it into the appconfig file for the Windows Application project (PTWin) At this point, there is no code in your data access component yet, so there are no new features to try out
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Calling a Stored Procedure
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If you de ned stored procedures for your application, the DAC needs a method that calls a stored procedure and returns the results of the procedure
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NOTE: Although this section refers to stored procedures, the code in this section works equally well with QueryDefs
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Looking back at the stored procedures created earlier in this chapter, you see that some stored procedures require parameters, like retrieve stored procedures that retrieve an item by ID, and save stored procedures Other stored procedures do not require parameters, such as retrieve
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stored procedures that retrieve all rows from a table To support this requirement, the DAC method that executes a stored procedure needs two overloads one with parameters and one without But what about the value returned from the DAC method At this point, you need to decide on the type of object to return from the method The NET Framework database library, ADONET, provides many choices Here are the three most common: DataReader is a read-only, forward-only stream of data from a database You access each row of data using the Read method of the DataReader DataTable is an in-memory cache of data that contains all the selected rows and columns When calling a stored procedure, the DataTable contains all the information de ned by a single SELECT statement DataSet is an in-memory cache of data that contains all the rows and columns in multiple selections of data When calling a stored procedure, the DataSet contains all of the information de ned by any number of SELECT statements The result of each SELECT statement is de ned as separate DataTable objects within the DataSet
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