LINQ to Entities Flow of Execution
Queries against the Entity Framework are represented by command tree queries, which execute against the object context. LINQ to Entities converts Language-Integrated Queries (LINQ) queries to command tree queries, executes the queries against the Entity Framework, and returns objects that can be used by both the Entity Framework and LINQ. The following is the process for creating and executing a LINQ to Entities query:
Construct an ObjectQuery instance from ObjectContext.
Compose a LINQ to Entities query in C# or Visual Basic by using the ObjectQuery instance.
Convert LINQ standard query operators and expressions to command trees.
Execute the query, in command tree representation, against the data source. Any exceptions thrown on the data source during execution are passed directly up to the client.
Return query results back to the client.
Constructing an ObjectQuery Instance
The ObjectQuery generic class represents a query that returns a collection of zero or more typed entities. An object query is typically constructed from an existing object context, instead of being manually constructed, and always belongs to that object context. This context provides the connection and metadata information that is required to compose and execute the query. The ObjectQuery generic class implements the IQueryable generic interface, whose builder methods enable LINQ queries to be incrementally built.
Composing the Query
Instances of the ObjectQuery generic class, which implements the generic IQueryable interface, serve as the data source for LINQ to Entities queries. In a query, you specify exactly the information that you want to retrieve from the data source. A query can also specify how that information should be sorted, grouped, and shaped before it is returned. In LINQ, a query is stored in a variable. This query variable takes no action and returns no data; it only stores the query information. After you create a query you must execute that query to retrieve any data.
LINQ to Entities queries can be composed in two different syntaxes: query expression syntax and method-based query syntax. Query expression syntax and method-based query syntax are new in C# 3.0 and Visual Basic 9.0.
For more information, see Queries in LINQ to Entities.
The following example in query expression syntax constructs an ObjectQuery instance from the AdventureWorks object context and uses Select to return all the rows from Product.
Using AWEntities As New AdventureWorksEntities
Dim products As ObjectQuery(Of Product) = AWEntities.Product
Dim productNames = _
From p In products _
Select p
End Using
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
ObjectQuery<Product> products = AWEntities.Product;
IQueryable<Product> productNames =
from p in products
select p;
}
The following example in method-based query syntax constructs an ObjectQuery instance from the AdventureWorks object context and uses Select to return all the rows from Product.
Using AWEntities As New AdventureWorksEntities
Dim productNames = AWEntities.Product.Select(Function(p) p.Name)
End Using
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
ObjectQuery<Product> products = AWEntities.Product;
IQueryable<Product> productNames = products.Select(p => p);
}
Query Conversion
To execute a LINQ to Entities query against the Entity Framework, the LINQ query must be converted to a command tree representation that can be executed against the Entity Framework.
LINQ to Entities queries are comprised of LINQ standard query operators (such as Select, Where, and GroupBy) and expressions (x > 10, Contact.LastName, and so on). LINQ operators are not defined by a class, but rather are methods on a class. In LINQ, expressions can contain anything allowed by types within the System.Linq.Expressions namespace and, by extension, anything that can be represented in a lambda function. This is a superset of the expressions that are allowed by the Entity Framework, which are by definition restricted to operations allowed on the database, and supported by ObjectQuery.
In the Entity Framework, both operators and expressions are represented by a single type hierarchy, which are then placed in a command tree. The command tree is used by the Entity Framework to execute the query. If the LINQ query cannot be expressed as a command tree, an exception will be thrown when the query is being converted. The conversion of LINQ to Entities queries involves two sub-conversions: the conversion of the standard query operators, and the conversion of the expressions.
There are a number of LINQ standard query operators that do not have a valid translation in LINQ to Entities. Attempts to use these operators will result in an exception at query translation time. For a list of supported LINQ to Entities operators, see Supported and Unsupported Methods (LINQ to Entities).
For more information about using the standard query operators in LINQ to Entities, see Standard Query Operators in LINQ to Entities Queries.
In general, expressions in LINQ to Entities are evaluated on the server, so the behavior of the expression should not be expected to follow CLR semantics. For more information, see Expressions in LINQ to Entities Queries.
Query Execution
After the LINQ query is created by the user, it is converted to a representation that is compatible with the Entity Framework (in the form of command trees), which is then executed against the data source. At query execution time, all query expressions (or components of the query) are evaluated on the client or on the server. This includes expressions that are used in result materialization or entity projections.
When a query expression is executed can vary. LINQ queries are executed each time the query variable is iterated over, not when the query variable is created; this is referred to as deferred execution. The query can also be forced to execute immediately, which is useful for caching query results. The following example uses Select to return all the rows from Product and display the product names. Iterating over the query variable in the foreach/For Each loop causes the query to execute.
Using AWEntities As New AdventureWorksEntities
Dim products As ObjectQuery(Of Product) = AWEntities.Product
Dim productNames = _
From p In products _
Select p.Name
Console.WriteLine("Product Names:")
For Each productName In productNames
Console.WriteLine(productName)
Next
End Using
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
ObjectQuery<Product> products = AWEntities.Product;
IQueryable<string> productNames =
from p in products
select p.Name;
Console.WriteLine("Product Names:");
foreach (var productName in productNames)
{
Console.WriteLine(productName);
}
}
When a LINQ to Entities query is executed, some expressions in the query might be executed on the server and some parts might be executed locally on the client. Client-side evaluation of an expression takes place before the query is executed on the server. If an expression is evaluated on the client, the result of that evaluation is substituted for the expression in the query, and the query is then executed on the server. Because queries are executed on the data source, the data source configuration overrides the behavior specified in the client. Null value handling and numerical precision are examples of this. Any exceptions thrown during query execution on the server are passed directly up to the client. For more information, see Query Execution.
Materialization
Materialization is the process of returning query results back to the client as CLR types. In LINQ to Entities, query results data records are never returned; there is always a backing CLR type, defined by the user or by the Entity framework, or generated by the compiler (anonymous types). All object materialization is performed by the Entity Framework. Any errors that result from an inability to map between the Entity Framework and the CLR will cause exceptions to be thrown during object materialization.
Query results are usually returned as one of the following:
A collection of zero or more typed entity objects or a projection of complex types in the EDM.
CLR types supported by the EDM.
Inline collections.
Anonymous types.
For more information, see Query Results.