In many scenarios you might want to reuse the queries across thread boundaries. In such cases, storing the compiled queries in static variables is especially effective. The following code example assumes a Queries class designed to store compiled queries, and assumes a Northwind class that represents a strongly typed DataContext.
Class Queries
Public Shared CustomersByCity As _
Func(Of Northwnd, String, IQueryable(Of Customer)) = _
CompiledQuery.Compile(Function(db As Northwnd, _
city As String) _
From c In db.Customers Where c.City = city Select c)
Public Shared CustomersById As _
Func(Of Northwnd, String, IQueryable(Of Customer)) = _
CompiledQuery.Compile(Function(db As Northwnd, _
id As String) _
db.Customers.Where(Function(c) c.CustomerID = id))
End Class
public static Func<Northwnd, string, IQueryable<Customer>>
CustomersByCity =
CompiledQuery.Compile((Northwnd db, string city) =>
from c in db.Customers where c.City == city select c);
public static Func<Northwnd, string, IQueryable<Customer>>
CustomersById = CompiledQuery.Compile((Northwnd db,
string id) => db.Customers.Where(c => c.CustomerID == id));
' The following example invokes such a compiled query in the main
' program
Public Function GetCustomersByCity(ByVal city As String) As _
IEnumerable(Of Customer)
Dim myDb = GetNorthwind()
Return Queries.CustomersByCity(myDb, city)
End Function
// The following example invokes such a compiled query in the main
// program.
public IEnumerable<Customer> GetCustomersByCity(string city)
{
var myDb = GetNorthwind();
return Queries.CustomersByCity(myDb, city);
}
You cannot currently store (in static variables) queries that return an anonymous type, because type has no name to provide as a generic argument. The following example shows how you can work around the issue by creating a type that can represent the result, and then use it as a generic argument.
Class SimpleCustomer
Private _ContactName As String
Public Property ContactName() As String
Get
Return _ContactName
End Get
Set(ByVal value As String)
_ContactName = value
End Set
End Property
End Class
Class Queries2
Public Shared CustomersByCity As Func(Of Northwnd, String, IEnumerable(Of SimpleCustomer)) = _
CompiledQuery.Compile(Of Northwnd, String, IEnumerable(Of SimpleCustomer))( _
Function(db As Northwnd, city As String) _
From c In db.Customers _
Where c.City = city _
Select New SimpleCustomer With {.ContactName = c.ContactName})
End Class
class SimpleCustomer
{
public string ContactName { get; set; }
}
class Queries2
{
public static Func<Northwnd, string, IEnumerable<SimpleCustomer>> CustomersByCity =
CompiledQuery.Compile<Northwnd, string, IEnumerable<SimpleCustomer>>(
(Northwnd db, string city) =>
from c in db.Customers
where c.City == city
select new SimpleCustomer { ContactName = c.ContactName });
}