It also knows how to add, update, and delete records when you send a DataSet back to the database. A DataSet is disconnected from the database and contains one or more DataTables depending on your query.
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It can also contain the relationships between the tables, so you can return less data and query the DataSet as if it were a mini database. ToString ds. This returns a DataTable object, which has a Rows collection property. To retrieve the value of a field, pass in either a name or an index to its indexer. Again, you are responsible for casting the value to the right type. A typical example of a scalar query is when you want to return a count of records in a table. Scalar queries perform better than DataReaders and DataSets because they return less data.
ToInt32 cmd. ExecuteScalar ; The trusty Command object is still used but the ExecuteScalar method is called, which returns an object.
You have to cast the object to the expected type. Be aware that if the query you execute returns more than one column or row, you will not get an exception. They will be ignored and you will be none the wiser, so be sure to write your queries correctly. SqlHelper The syntax for connecting to a database and executing queries can become quite repetitive.
A good practice is to wrap these calls into a single class that the DAL can easily call with typed parameters.
Luckily, Microsoft already did this. From their website, you can download the Microsoft Data Access Application Block, which contains a class called SqlHelper that is a wrapper for all of the aforementioned methods but contains overloaded methods of each depending on how you implement the DAL. Later versions use generics and can be used against different databases such as SQL Server and Oracle, but the code is much more complex. This provides the calling procedure with greater flexibility because you may need to execute the same stored procedure but it may be part of a transaction in one call and by itself in another call.
The first, fourth, and seventh methods call the second, fifth, and eighth methods, respectively. The only difference is that it passes in null for the SqlParameters argument. When calling the second, fifth, or eighth method, you need to know the parameters for the stored procedure and create them before calling the method. This involves an extra hit to the database because it queries the database, but the SqlHelper class caches the parameter list the first time the procedure is called and then uses the cached parameters for all subsequent calls. After the SqlParameters have been resolved, they call the second, fifth, and eighth methods, which take a SqlParameters array, so all calls eventually make it to either the second, fifth, or eighth methods.
This way, I avoid the extra hit to the database, I can explicitly create the parameter and assign its value, unlike when calling the methods with the object array, and I can use output parameters and retrieve their values. When adding records, you normally return the ID of the newly added record. Output; SqlHelper. ToInt32 parameters. If a user pulls up a record on the screen, looks at it for a minute and makes a change and saves the record back to the database, the system should only update the record if it was the same as when the user started looking at it.
To accomplish this, you can add a datetime field or an int field that is incremented each time a record is updated. When you retrieve the record from the database, you need to persist this value somewhere on the page and pass it back to the database when saving the record. The update procedure would include this value in the where clause and only update the record if the value matches what it was when the record was 19 c The TimeStamp field is discussed in more detail later in this chapter.
Use the UserAccount sample shown earlier as an example. ToInt32 SqlHelper. If the rows affected do not equal 1, then the method returns false and the business layer can handle it appropriately. I typically do not throw exceptions here because this is a special case that I expect to handle.
Exceptions can be expensive and they should be used only for an unknown condition. Up until now, ADO. NET objects from the developer and makes it even easier to communicate with the database. After explaining the options most people have been debating about, the solution proposes a model that uses LINQ to SQL, which is a good compromise between the options. The method for getting data to and from your business objects is irrelevant to the user interface, but your choice can affect how the data gets to and from your database.
DataReaders are extremely quick and are great for a one-time pass of the data for loading objects. DataReaders are connected to the database and are forward-only. A disadvantage of the DataReader is that it is connected to the database and the caller is responsible for closing and releasing the connection back to the connection pool. This might not be a problem for small application teams, but on larger teams where different people work on different layers, you need to trust that everyone knows to close the connection.
Another disadvantage of the DataReader is that it is only a one-way communication vehicle for passing data back to the caller. By definition, a DataReader is read-only, so if you choose the DataReader as the vehicle for passing data back to the caller, you need a different vehicle for passing data into the DAL. Another disadvantage to the DataReader is that it is not strongly typed and each field must be cast to its correct type by the user of the DataReader. The second option is the DataSet. The DataSet is an in-memory representation of the tables in the database.
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The DataSet is completely disconnected from the data source, which could be a database or any other source such as an XML document. DataSets also have built-in support for adding, updating, and deleting records in the database. They also handle optimistic concurrency control and enable the caller to manage issues accordingly.
You can also represent complex data structures in a DataSet and represent relational data between DataTables with DataRelation objects. This helps limit the data passed through the layers. Maintenance can then become an issue. DataSets are also more expensive than DataReaders because they return more information about the underlying DataTables.
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This can be an issue if you are marshaling objects across tiers. The third option is the typed DataSet. Typed DataSets were all the rage when the. NET Framework was first released. I can remember watching demo after demo demonstrating how easy it was to create a typed DataSet and bind it to the DataGrid. By the way, whatever happened to the DataGrid? Anyway, a typed DataSet uses a DataSet for its underlying data but uses strongly typed properties to access the fields, which makes it easier for the caller to access the data. This makes reading code much easier and it enables you to use IntelliSense when developing your application.
Typed DataSets also provide compile-time checking, rather than runtime checking, for their properties. Anytime you can use the compiler to find an exception, rather than the runtime, it is usually worth it.
A disadvantage of typed DataSets is that they are difficult to maintain. The tool that is used to generate a typed DataSet creates a class that you add to your project. You can then customize that class to add business rules or calculated fields, but if you change the underlying table you need to either regenerate the class, which causes you to lose all your custom code, or manually update the class yourself.
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The fourth option is custom entity objects. Custom entity objects are custom-built classes that mimic the relational structure of the database. You usually create properties for each field in the underlying table and represent more than one record using a generic List object or an ArrayList. This enables the DAL to remain in control of the connection, which is a good thing.
You could also use structs, rather than classes, to represent the data, as structs are lighter than classes because they are value types, whereas classes are reference types.
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