asp.net c# print barcode Advanced LINQ to SQL features in C#.NET

Generate Quick Response Code in C#.NET Advanced LINQ to SQL features

Advanced LINQ to SQL features
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Notice this time, since we already assigned the Base Class Discriminator, Discriminator Property, and Inheritance Default, these values are retained. All we need to do is set the Derived Class Code. Since we want AuthorUsers loaded when the type is A, enter the value A. With the inheritance mappings set, save the changes to the designer. Viewing the generated code in the .designer.cs file shows the attributes that were added to your class definition.
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[InheritanceMapping(Code="S", Type=typeof(UserBase), IsDefault=true)] [InheritanceMapping(Code="A", Type=typeof(AuthorUser))] [InheritanceMapping(Code="P", Type=typeof(PublisherUser))] [Table(Name="dbo.[User]")] Public partial class UserBase { //Implementation code omitted }
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The runtime relies on the attributes (or the XML mapping file) to indicate which objects to load. In addition to the InheritanceMapping attributes defined on the class, there is one other parameter we set when we made the changes in the designer. The attribute in question is not on the table, but on the UserType s Column attribute. Note that the IsDiscriminator parameter is now set to true.
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[Column(Storage="_UserType", IsDiscriminator=true)] public char UserType
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As we fetch records, LINQ to SQL will check the property decorated by the IsDiscriminator attribute. It will then compare the underlying value against the list of InheritanceMapping codes in the base class to determine which type to instantiate. With this value set, we ve completed the necessary steps to create our inheritance trees. Before we consume our new inherited objects, let s use the designer to establish one more set of relationships. At this point, we can use the designer to assign associations between the Publisher and Author objects and our newly customized AuthorUser and PublisherUser objects. This will allow us to drill into the object hierarchies even though we don t have foreign key indexes associated between the tables in our database. The designer makes adding this functionality a breeze. Select the Association tool from the toolbox. Click on the ID property of the Author class, followed by the AuthorId property of the AuthorUser. Do the same for the Publisher and PublisherUser. The designer should now look something like the illustration we started with in figure 8.4. We re done with the changes to our designer. Let s move on to actually using our hard work. Listing 8.28 demonstrates several ways we can access our user lists.
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Listing 8.28 Consuming inherited LINQ to SQL objects
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var query = from user in context.UserBases select user.Name; var authors = from user in context.UserBases where user is AuthorUser select user.Name; var publishers = from user in context.UserBases.OfType<PublisherUser>() select user.Name;
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In the first example B, we can select all users from our user table regardless of the implementing type. We can iterate over the list even though each row is represented by a different object type because each of the objects inherits from the same UserBase class. Here is the SQL that is generated for this first query:
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SELECT [t0].[Name] FROM [dbo].[User] AS [t0]
In the second query, we limit the records we want to retrieve based on the object type. To fetch just the AuthorUser objects (where the UserType=A) we can use strong typing and specify we only want users where the implementing type is AuthorUser in the where clause C. The final example illustrates another method of filtering. In this case, we use the OfType extension method to retrieve only objects that match the PublisherUser type D. Since we re selecting just the user name, both of these queries result in the same generated SQL:
SELECT [t0].[Name] FROM [dbo].[User] AS [t0] WHERE [t0].[UserType] = @p0
One thing to keep in mind, if you have columns that apply to some derived classes but not others, they need to be marked as Nullable in the database. Otherwise, updating values in the table on objects that don t implement those properties will throw an exception because the values were not supplied. We ve only scratched the surface of the capabilities that inheritance can offer. As long as the data is limited to a single table, implementing inheritance with LINQ to SQL is relatively painless. The InheritanceMapping attributes on the base class and Discriminator column on the table are all that we need. The inheritance model supported by LINQ to SQL does suffer from a couple of weaknesses. First, the runtime requires that the base class of the object hierarchy
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