c# net qr code generator SELECT o FROM AdventureWorksLT2008Entities.SalesOrderHeaders AS o WHERE o.OrderDate = @orderDate in Visual C#.NET

Paint QR Code 2d barcode in Visual C#.NET SELECT o FROM AdventureWorksLT2008Entities.SalesOrderHeaders AS o WHERE o.OrderDate = @orderDate

SELECT o FROM AdventureWorksLT2008Entities.SalesOrderHeaders AS o WHERE o.OrderDate = @orderDate
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the query will still return IDataRecord objects, not entities. Each data record returned by this query would have a single column called o that contains a SalesOrderHeader entity. To get to the entity you d need to unwrap it inside your loop:
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foreach (var row in query) { SalesOrderHeader o = (SalesOrderHeader) row["o"]; Console.WriteLine(o.TotalDue); }
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The VALUE keyword is just a shortcut that tells ESQL to omit the IDataRecord wrapper, and to return a sequence of unwrapped entities. This enables Example 14-12 to assume that it will get SalesOrderHeader entities back from the query.
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Mixing ESQL and LINQ
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LINQ to Entities and ESQL are not mutually exclusive. You are free to use an ESQL query as the source for a LINQ query. Here s a contrived example:
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var orders = dbContext.CreateQuery<SalesOrderHeader>("SELECT VALUE o " + "FROM AdventureWorksLT2008Entities.SalesOrderHeaders AS o " + "WHERE o.OrderDate = @orderDate", new ObjectParameter("orderDate", orderDate)); var orderedOrders = from order in orders orderby order.DueDate select order;
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This might be useful if you wanted to store ESQL queries in some sort of configuration mechanism to allow the exact query to be changed, but to do further processing of the results of that query with LINQ.
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The EntityClient ADO.NET Provider
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Yet another feature enabled by ESQL is that it lets code built around the v1 ADO.NET mechanisms shown in Example 14-1 work with the EF. The System.Data.Entity Client namespace defines concrete types that derive from the abstract base classes listed in Table 14-1: EntityConnection derives from DbConnection, EntityCommand derives from DbCommand, and so on. As far as code written to use these abstract base classes is concerned, the Entity Framework ends up looking like just another database with another funky variety of SQL. As long as your ESQL selects only column values and not whole entities, queries will only ever return the same basic data types other providers would, so the behavior will look much like any other ADO.NET v1 provider.
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Object Context
As you ve seen, the object context provides access to entities. For each entity we define in our EDM, the generated object context class provides a property that we can use as the source for a LINQ query. We ve also used its CreateQuery<T> method to build ESQLbased queries. The object context provides some other services.
Connection Handling
To execute database queries, it s necessary to connect to a database, so the object context needs connection information. This information typically lives in the App.config file when you first run the EDM wizard, it will add a configuration file if your application does not already have one, and then it adds a connection string. Example 14-13 shows a configuration file containing a typical Entity Framework connection string. (This has been split over multiple lines to fit normally the connectionString attribute is all on one line.)
<configuration> <connectionStrings> <add name="AdventureWorksLT2008Entities" connectionString="metadata=res://*/AdventureWorksModel.csdl| res://*/AdventureWorksModel.ssdl|res://*/AdventureWorksModel.msl; provider=System.Data.SqlClient;provider connection string= "Data Source=.\sqlexpress;Initial Catalog=AdventureWorksLT2008; Integrated Security=True;MultipleActiveResultSets=True"" providerName="System.Data.EntityClient" /> </connectionStrings> </configuration>
This is a rather more complex connection string than the one we saw back in Example 14-1, because the Entity Framework needs three things in its connection string: information on where to find the EDM definition, the type of underlying database provider to use, and the connection string to pass to that underlying provider. This last
part an ordinary SQL Server connection string, enclosed in " character entities is highlighted in Example 14-13 in bold. The three URIs in the metadata section of the connectionString the ones beginning with res:// point to the three parts of the EDM: the conceptual schema, the storage schema, and the mappings. Visual Studio extracts these from the .edmx file and embeds them as three XML resource streams in the compiled program. Without these, the EF wouldn t know what the conceptual and storage schemas are supposed to look like, or how to map between them.
It may seem a bit weird for the locations of these EDM resources to be in a connection string. It might seem more natural for the XML to use a separate attribute for each one. However, as you ve seen, the System.Data.EntityClient namespace conforms to the ADO.NET v1 model so that it s possible for old-style data access code to perform queries against the EDM. Since the ADO.NET v1 model includes an assumption that it s possible to put all the information defining a particular data source into a single connection string, the Entity Framework has to follow suit. And since the EF cannot function without the XML EDM definitions, the connection string has to say where those live.
After the EDM metadata resources, you can see a provider property, which in Example 14-13 indicates that the underlying database connection is to be provided by the SQL Server client. The EF passes the provider connection string on to that provider. You don t have to use the App.config to configure the connection. The object context offers a constructor overload that accepts a connection string. The configuration file is useful it s where the object context s no-parameters constructor we ve been using in the examples gets its connection information from but what if you want to let just the underlying database connection string be configurable, while keeping the parts of the connection string identifying the EDM resources fixed Example 14-14 shows how you could achieve this. It retrieves the configured values for these two pieces and uses the EntityConnectionStringBuilder helper to combine this with the EDM resource locations, forming a complete EF connection string.
using System.Configuration; using System.Data.EntityClient; ... // Retrieve the connection string for the underlying database provider. ConnectionStringSettings dbConnectionInfo = ConfigurationManager.ConnectionStrings["AdventureWorksSql"]; var csb = new EntityConnectionStringBuilder();
csb.Provider = dbConnectionInfo.ProviderName; csb.ProviderConnectionString = dbConnectionInfo.ConnectionString; csb.Metadata = "res://*/AdventureWorksModel.csdl|" + "res://*/AdventureWorksModel.ssdl|res://*/AdventureWorksModel.msl"; using (var dbContext = new AdventureWorksLT2008Entities(csb.ConnectionString)) { ... }
This code uses the ConfigurationManager in the System.Configuration namespace, which provides a ConnectionStrings property. (This is in a part of the .NET Framework class library that s not referenced by default in a .NET console application, so we need to add a reference to the System.Configuration component for this to work.) This provides access to any connection strings in your App.config file; it s the same mechanism the EF uses to find its default connection string. Now that Example 14-14 is providing the EDM resources in code, our configuration file only needs the SQL Server part of the connection string, as shown in Example 14-15 (with a long line split across multiple lines to fit). So when the application is deployed, we have the flexibility to configure which database gets used, but we have removed any risk that such a configuration change might accidentally break the references to the EDM resources.
<configuration> <connectionStrings> <add name="AdventureWorksSql" providerName="System.Data.SqlClient" connectionString="Data Source=.\sqlexpress; Initial Catalog=AdventureWorksLT2008; Integrated Security=True;MultipleActiveResultSets=True" /> </connectionStrings> </configuration>
Besides being able to change the connection information, what else can we do with the connection We could choose to open the connection manually we might want to verify that our code can successfully connect to the database. But in practice, we don t usually do that the EF will connect automatically when we need to. The main reason for connecting manually would be if you wanted to keep the connection open across multiple requests if the EF opens a connection for you it will close it again. In any case, we need to be prepared for exceptions anytime we access the database being able to connect successfully is no guarantee that someone won t trip over a network cable at some point between us manually opening the connection and attempting to execute a query. So in practice, the connection string is often the only aspect of the connection we need to take control of.
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